Fix `$rails_rake_task` global variable warning without replacing the value of $rails_rake_task if it is already set.
Fix `$rails_rake_task` global variable warning without replacing the value of $rails_rake_task if it is already set.

Fixes #4591.
esparta/rails,travisofthenorth/rails,flanger001/rails,assain/rails,aditya-kapoor/rails,Erol/rails,mohitnatoo/rails,yasslab/railsguides.jp,lcreid/rails,Spin42/rails,Stellenticket/rails,Stellenticket/rails,gcourtemanche/rails,sealocal/rails,iainbeeston/rails,rbhitchcock/rails,kddeisz/rails,sergey-alekseev/rails,georgeclaghorn/rails,deraru/rails,assain/rails,maicher/rails,mohitnatoo/rails,notapatch/rails,yhirano55/rails,starknx/rails,rails/rails,Vasfed/rails,mtsmfm/railstest,baerjam/rails,kachick/rails,gcourtemanche/rails,kddeisz/rails,pvalena/rails,iainbeeston/rails,fabianoleittes/rails,kachick/rails,xlymian/rails,riseshia/railsguides.kr,utilum/rails,repinel/rails,Erol/rails,yalab/rails,Stellenticket/rails,arunagw/rails,mohitnatoo/rails,bogdanvlviv/rails,gavingmiller/rails,mijoharas/rails,illacceptanything/illacceptanything,vassilevsky/rails,Sen-Zhang/rails,starknx/rails,MSP-Greg/rails,kenta-s/rails,MSP-Greg/rails,tjschuck/rails,coreyward/rails,kmcphillips/rails,vipulnsward/rails,mtsmfm/rails,esparta/rails,betesh/rails,riseshia/railsguides.kr,stefanmb/rails,sergey-alekseev/rails,rails/rails,richseviora/rails,brchristian/rails,Envek/rails,felipecvo/rails,mtsmfm/rails,tijwelch/rails,ttanimichi/rails,voray/rails,alecspopa/rails,mohitnatoo/rails,tgxworld/rails,travisofthenorth/rails,samphilipd/rails,koic/rails,eileencodes/rails,pvalena/rails,hanystudy/rails,mathieujobin/reduced-rails-for-travis,tjschuck/rails,palkan/rails,printercu/rails,notapatch/rails,printercu/rails,mechanicles/rails,illacceptanything/illacceptanything,georgeclaghorn/rails,utilum/rails,assain/rails,baerjam/rails,brchristian/rails,jeremy/rails,EmmaB/rails-1,schuetzm/rails,gauravtiwari/rails,untidy-hair/rails,rbhitchcock/rails,kachick/rails,illacceptanything/illacceptanything,baerjam/rails,eileencodes/rails,MSP-Greg/rails,bolek/rails,assain/rails,Envek/rails,joonyou/rails,aditya-kapoor/rails,illacceptanything/illacceptanything,illacceptanything/illacceptanything,kenta-s/rails,ttanimichi/rails,Envek/rails,yalab/rails,joonyou/rails,kamipo/rails,arjes/rails,matrinox/rails,odedniv/rails,yahonda/rails,xlymian/rails,yawboakye/rails,xlymian/rails,gavingmiller/rails,starknx/rails,gavingmiller/rails,pvalena/rails,utilum/rails,yalab/rails,deraru/rails,bogdanvlviv/rails,ngpestelos/rails,georgeclaghorn/rails,richseviora/rails,amoody2108/TechForJustice,mtsmfm/rails,travisofthenorth/rails,ngpestelos/rails,pvalena/rails,georgeclaghorn/rails,schuetzm/rails,illacceptanything/illacceptanything,vassilevsky/rails,tgxworld/rails,mijoharas/rails,tjschuck/rails,esparta/rails,voray/rails,repinel/rails,stefanmb/rails,betesh/rails,alecspopa/rails,Spin42/rails,utilum/rails,marklocklear/rails,sealocal/rails,Erol/rails,yhirano55/rails,brchristian/rails,bogdanvlviv/rails,rossta/rails,gauravtiwari/rails,arjes/rails,Vasfed/rails,vipulnsward/rails,deraru/rails,88rabbit/newRails,Sen-Zhang/rails,kaspth/rails,rafaelfranca/omg-rails,mathieujobin/reduced-rails-for-travis,Spin42/rails,yawboakye/rails,ledestin/rails,fabianoleittes/rails,illacceptanything/illacceptanything,odedniv/rails,ledestin/rails,maicher/rails,Edouard-chin/rails,yasslab/railsguides.jp,coreyward/rails,yawboakye/rails,pschambacher/rails,voray/rails,schuetzm/rails,gauravtiwari/rails,vassilevsky/rails,eileencodes/rails,printercu/rails,sealocal/rails,prathamesh-sonpatki/rails,betesh/rails,schuetzm/rails,deraru/rails,yahonda/rails,hanystudy/rails,rossta/rails,coreyward/rails,gcourtemanche/rails,MichaelSp/rails,yalab/rails,yhirano55/rails,yahonda/rails,matrinox/rails,mtsmfm/railstest,palkan/rails,richseviora/rails,felipecvo/rails,lucasmazza/rails,alecspopa/rails,kddeisz/rails,notapatch/rails,vipulnsward/rails,kamipo/rails,amoody2108/TechForJustice,arunagw/rails,rails/rails,elfassy/rails,bogdanvlviv/rails,shioyama/rails,yasslab/railsguides.jp,kirs/rails-1,illacceptanything/illacceptanything,jeremy/rails,elfassy/rails,Vasfed/rails,mechanicles/rails,eileencodes/rails,kmayer/rails,arjes/rails,bradleypriest/rails,riseshia/railsguides.kr,repinel/rails,Edouard-chin/rails,travisofthenorth/rails,stefanmb/rails,kaspth/rails,rafaelfranca/omg-rails,shioyama/rails,baerjam/rails,untidy-hair/rails,prathamesh-sonpatki/rails,sergey-alekseev/rails,yahonda/rails,iainbeeston/rails,vipulnsward/rails,mtsmfm/railstest,illacceptanything/illacceptanything,marklocklear/rails,gfvcastro/rails,felipecvo/rails,Erol/rails,tijwelch/rails,matrinox/rails,betesh/rails,joonyou/rails,arunagw/rails,esparta/rails,illacceptanything/illacceptanything,lcreid/rails,koic/rails,Edouard-chin/rails,koic/rails,illacceptanything/illacceptanything,lucasmazza/rails,pschambacher/rails,MichaelSp/rails,hanystudy/rails,mathieujobin/reduced-rails-for-travis,palkan/rails,EmmaB/rails-1,prathamesh-sonpatki/rails,aditya-kapoor/rails,flanger001/rails,gfvcastro/rails,riseshia/railsguides.kr,BlakeWilliams/rails,notapatch/rails,illacceptanything/illacceptanything,EmmaB/rails-1,yawboakye/rails,88rabbit/newRails,kirs/rails-1,prathamesh-sonpatki/rails,ttanimichi/rails,illacceptanything/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