Update prebuilt.py call to use the _proper_ variable for finding the build root.
Update prebuilt.py call to use the _proper_ variable for finding the build root.

BUG=8603
TEST=Tested where GCLIENT_ROOT points to on rm and x86

Review URL: http://codereview.chromium.org/4363001

nicescale/scripts,gtank/scripts,bcwaldon/coreos-scripts,coreos/scripts,gtank/scripts,trnubo/scripts,vmware/coreos-scripts,crawford/scripts,BugRoger/scripts,gtank/scripts,smilart/scripts,bcwaldon/coreos-scripts,mjg59/scripts,BugRoger/scripts,fivethreeo/scripts,marineam/coreos-scripts,andrejro/coreos--scripts,endocode/scripts,coreos/scripts,mischief/scripts,mjg59/scripts,glevand/coreos--scripts,zhang0137/scripts,endocode/scripts,smilart/scripts,marineam/coreos-scripts,crawford/scripts,endocode/scripts,nicescale/scripts,mischief/scripts,trnubo/scripts,marineam/coreos-scripts,vmware/coreos-scripts,nicescale/scripts,andrejro/coreos--scripts,zhang0137/scripts,glevand/coreos--scripts,sigma/coreos-scripts,sigma/coreos-scripts,BugRoger/scripts,bcwaldon/coreos-scripts,vmware/coreos-scripts,mischief/scripts,fivethreeo/scripts,trnubo/scripts,andrejro/coreos--scripts,fivethreeo/scripts,crawford/scripts,smilart/scripts,mjg59/scripts,zhang0137/scripts,sigma/coreos-scripts