cmd/go: TestExecBuildX is too sensitive to GOROOT #29004
Description
When making changes to cmd/go (and not other parts of the standard library or toolchain), most of the test cases in go test cmd/go continue to pass without rebuilding. However, TestExecBuildX seems to fail pretty reliably unless make.bash has been run at the exact commit being tested.
That's a bit annoying, because make.bash itself is pretty slow. I'd like to figure out whether we can make TestExecBuildX more robust to changes.