Modernize the code to use sync.OnceValue[s] instead of sync.Once.
While at it, reuse the result of exec.LookPath("go") in tryGoBuild.
Change-Id: I13eff3dd55797846680e506fffb7c49c8296829d
Reviewed-on: https://go-review.googlesource.com/c/go/+/609796
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org> Reviewed-by: Alan Donovan <adonovan@google.com>
Commit-Queue: Ian Lance Taylor <iant@golang.org> Reviewed-by: Ian Lance Taylor <iant@google.com>