Separate kernel and syzkaller build failures. Fix logic to understand when a build is fixed: look if kernel/syzkaller commit changes to understand if it's a new good build or re-upload of an old build. Fixes #1014
PrependContext prepends the string rather than format.
GOPATH/GOROOT are also needed for jobs. Updater can be disabled with -autoupdate=0, so move GOPATH/GOROOT setup to main. Update #501
