make.bash: preserve GOROOT_BOOTSTRAP
CL 582076 made the GOROOT_BOOTSTRAP set to "", which in turn
causes the next iteration to return true. "$GOROOT_BOOTSTRAP/bin/go"
becomes "/bin/go", which always exists.
Fixes #67654
Change-Id: I290bd4a7defc58e4b34d5bf955ef4f34569ff0ed
GitHub-Last-Rev:
9c7134fa44874b0a9d03807aa583b1088cc13922
GitHub-Pull-Request: golang/go#67655
Reviewed-on: https://go-review.googlesource.com/c/go/+/588436
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>