don't exec in js:build rake task
don't exec in js:build rake task

this is so evil

test plan:
  * run rake canvas:compile_assets, and verify that the api
    documentation gets built after js:build runs

Change-Id: If931f39735855b13e09ee773f296f24b555b3dc1
Reviewed-on: https://gerrit.instructure.com/9096
Reviewed-by: Ryan Florence <2d8900489f91ac272fd9ce9d5789db6522668ca2@instructure.com>
Tested-by: Brian Palmer <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@instructure.com>

landonwilkins/canvas-lms,greyhwndz/canvas-lms,wye0220/shindig-canvas,samuels410/greatlakes-lms,leftplusrightllc/canvas-lms,venturehive/canvas-lms,va7map/canvas-lms,skmezanul/canvas-lms,HotChalk/canvas-lms,leftplusrightllc/canvas-lms,merichar/canvas-lms,SwinburneOnline/canvas-lms,HotChalk/canvas-lms,nditech/red-innovadores-lms,efrj/canvas-lms,arrivu/arrivu-lms,em2/canvas-lms,grahamb/canvas-lms,Unow/ins,tgroshon/canvas-lms,leftplusrightllc/canvas-lms,djbender/canvas-lms,antoniuslin/canvas-lms,grahamb/canvas-lms,ottenhoff/canvas-lms,skmezanul/canvas-lms,jay3126/canvas-lms,samuels410/greatlakes-lms,redconfetti/canvas-lms,Rvor/canvas-lms,landonwilkins/canvas-lms,HotChalk/canvas-lms,leftplusrightllc/canvas-lms,venturehive/canvas-lms,loganrun/pincan,kyroskoh/canvas-lms,juoni/canvas-lms,arrivu/hoodemo,arrivu/hoodemo,rup/Tarrax,sfu/canvas-lms,rup/Tarrax,fronteerio/canvas-lms,wye0220/shindig-canvas,tgroshon/canvas-lms,em2/canvas-lms,samuels410/greatlakes-lms,fronteerio/canvas-lms,wye0220/shindig-canvas,redconfetti/canvas-lms,greyhwndz/canvas-lms,samuels410/greatlakes-lms,sfu/canvas-lms,Rvor/canvas-lms,merichar/canvas-lms,Jyaasa/canvas-lms,arrivu/qibla-ecampus,efrj/canvas-lms,nditech/red-innovadores-lms,SwinburneOnline/canvas-lms,tgroshon/canvas-lms,kyroskoh/canvas-lms,sfu/canvas-lms,ankithbti/canvas_lms,rup/Tarrax,va7map/canvas-lms,arrivu/arrivu-lms,roxolan/canvas-lms,greyhwndz/canvas-lms,nditech/red-innovadores-lms,parbros/red-innovadores-lms,arrivu/jigsaw-lms,usmschuck/canvas,juoni/canvas-lms,ShreniiNepal/shrenii_lmp,ShreniiNepal/shrenii_lmp,ShreniiNepal/shrenii_lmp,Rvor/canvas-lms,sfu/canvas-lms,arrivu/hoodemo,sdeleon28/canvas-lms,sli7246/canvas_branch,landonwilkins/canvas-lms,ottenhoff/canvas-lms,djbender/canvas-lms,em2/canvas-lms,sli7246/canvas_branch,va7map/canvas-lms,efrj/canvas-lms,skmezanul/canvas-lms,arrivu/qibla-ecampus,ankithbti/canvas_lms,arrivu/arrivu-lms,Unow/canvas-lms,antoniuslin/canvas-lms,arrivu/jigsaw-lms,roxolan/canvas-lms,fronteerio/canvas-lms,Jyaasa/canvas-lms,Rvor/canvas-lms,arrivu/jigsaw-lms,ottenhoff/canvas-lms,kyroskoh/canvas-lms,kyroskoh/canvas-lms,wrdsb/canvas-lms,utkarshx/canvas-lms,redconfetti/canvas-lms,sdeleon28/canvas-lms,djbender/canvas-lms,greyhwndz/canvas-lms,juoni/canvas-lms,roxolan/canvas-lms,sfu/canvas-lms,arrivu/qibla-ecampus,whalejasmine/canvas-lms,shidao-fm/canvas-lms,utkarshx/canvas-lms,HotChalk/canvas-lms,parbros/red-innovadores-lms,skmezanul/canvas-lms,shidao-fm/canvas-lms,venturehive/canvas-lms,wrdsb/canvas-lms,wimemx/Canvas,Unow/canvas-lms,wimemx/Canvas,Unow/canvas-lms,SwinburneOnline/canvas-lms,antoniuslin/canvas-lms,nditech/red-innovadores-lms,shidao-fm/canvas-lms,loganrun/pincan,whalejasmine/canvas-lms,shidao-fm/canvas-lms,va7map/canvas-lms,Unow/ins,djbender/canvas-lms,parbros/red-innovadores-lms,grahamb/canvas-lms,jay3126/canvas-lms,sdeleon28/canvas-lms,merichar/canvas-lms,grahamb/canvas-lms,wrdsb/canvas-lms,sli7246/canvas_branch,utkarshx/canvas-lms,usmschuck/canvas,tgroshon/canvas-lms,juoni/canvas-lms,Jyaasa/canvas-lms,Unow/ins,fronteerio/canvas-lms,whalejasmine/canvas-lms,usmschuck/canvas,wimemx/Canvas,loganrun/pincan,utkarshx/canvas-lms,antoniuslin/canvas-lms,Jyaasa/canvas-lms,roxolan/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,ankithbti/canvas_lms,usmschuck/canvas,jay3126/canvas-lms,venturehive/canvas-lms,efrj/canvas-lms,jay3126/canvas-lms,whalejasmine/canvas-lms,ShreniiNepal/shrenii_lmp,landonwilkins/canvas-lms,wrdsb/canvas-lms,grahamb/canvas-lms,SwinburneOnline/canvas-lms,sdeleon28/canvas-lms,redconfetti/canvas-lms