os: use sync.OnceValue
Simplify the code and reduce global variables.
Change-Id: Id322836e8b6b6c4434136b95700ed4070ba52300
GitHub-Last-Rev:
7c9d409855c682b6de1f338f69d9193a7887f9f0
GitHub-Pull-Request: golang/go#69962
Reviewed-on: https://go-review.googlesource.com/c/go/+/621456
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>