builder/googlecompute: Revert to using default service account
builder/googlecompute: Revert to using default service account

Fixes issue with cross Project builds.

Closes #4608

Regression introduced in PR #3639

smaato/packer,boumenot/packer,tb3088/packer,zhuzhih2017/packer,jen20/packer,smaato/packer,jen20/packer,rnaveiras/packer,p1-nico/packer,markpeek/packer,bryson/packer,KohlsTechnology/packer,bryson/packer,tas50/packer,routelastresort/packer,hashicorp/packer,boumenot/packer,pieter-lazzaro/packer,ChrisLundquist/packer,grubernaut/packer,tas50/packer,smaato/packer,rnaveiras/packer,tsmolka/packer,pecigonzalo/packer,timsutton/packer,zhuzhih2017/packer,hashicorp/packer,atlassian/packer,pvandervelde/packer,c22/packer,ricardclau/packer,tsmolka/packer,atlassian/packer,tsmolka/packer,KohlsTechnology/packer,tb3088/packer,ricardclau/packer,mitchellh/packer,pieter-lazzaro/packer,pieter-lazzaro/packer,zhuzhih2017/packer,c22/packer,p1-nico/packer,tsmolka/packer,routelastresort/packer,dayglojesus/packer,pecigonzalo/packer,tas50/packer,markpeek/packer,mkuzmin/packer,boumenot/packer,tb3088/packer,tsmolka/packer,youhong316/packer,taliesins/packer,markpeek/packer,grubernaut/packer,boumenot/packer,ChrisLundquist/packer,p1-nico/packer,zhuzhih2017/packer,KohlsTechnology/packer,grubernaut/packer,jen20/packer,sean-/packer,sean-/packer,c22/packer,atlassian/packer,arizvisa/packer,tsmolka/packer,markpeek/packer,quantang/packer,sean-/packer,bryson/packer,zhuzhih2017/packer,dayglojesus/packer,tb3088/packer,ricardclau/packer,KohlsTechnology/packer,KohlsTechnology/packer,arizvisa/packer,taliesins/packer,rnaveiras/packer,routelastresort/packer,tas50/packer,ricardclau/packer,timsutton/packer,KohlsTechnology/packer,smaato/packer,tas50/packer,hashicorp/packer,p1-nico/packer,dave2/packer,quantang/packer,paulmey/packer,mkuzmin/packer,dayglojesus/packer,mkuzmin/packer,ChrisLundquist/packer,dave2/packer,c22/packer,pvandervelde/packer,ricardclau/packer,timsutton/packer,dave2/packer,p1-nico/packer,arizvisa/packer,dayglojesus/packer,grubernaut/packer,pecigonzalo/packer,youhong316/packer,youhong316/packer,dayglojesus/packer,tas50/packer,rnaveiras/packer,smaato/packer,paulmey/packer,pvandervelde/packer,hashicorp/packer,mkuzmin/packer,taliesins/packer,pieter-lazzaro/packer,atlassian/packer,markpeek/packer,youhong316/packer,dave2/packer,paulmey/packer,mitchellh/packer,boumenot/packer,sean-/packer,mitchellh/packer,rnaveiras/packer,arizvisa/packer,youhong316/packer,grubernaut/packer,paulmey/packer,p1-nico/packer,tb3088/packer,grubernaut/packer,markpeek/packer,pvandervelde/packer,c22/packer,pvandervelde/packer,dave2/packer,bryson/packer,quantang/packer,ChrisLundquist/packer,ricardclau/packer,bryson/packer,pieter-lazzaro/packer,jen20/packer,mkuzmin/packer,dave2/packer,quantang/packer,taliesins/packer,taliesins/packer,routelastresort/packer,pieter-lazzaro/packer,paulmey/packer,timsutton/packer,arizvisa/packer,youhong316/packer,arizvisa/packer,zhuzhih2017/packer,ChrisLundquist/packer,atlassian/packer,atlassian/packer,pecigonzalo/packer,taliesins/packer,mitchellh/packer,tb3088/packer,paulmey/packer,sean-/packer,mkuzmin/packer,timsutton/packer,quantang/packer,pvandervelde/packer,sean-/packer,ChrisLundquist/packer,c22/packer,rnaveiras/packer,bryson/packer,jen20/packer,routelastresort/packer,jen20/packer,mitchellh/packer,smaato/packer,timsutton/packer