cmd/go/internal/auth: use sync.OnceValues

cmd/go/internal/auth: use sync.OnceValues

Use sync.OnceValues (CL 451356, since Go 1.21) instead of sync.Once for
cleaner code and less global variables, preventing their potential
misuse.

Change-Id: I9d7ccc42847fe77af1757672c31bb39e20007f92
Reviewed-on: https://go-review.googlesource.com/c/go/+/611016
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>