os: check relative paths in UserConfigDir and UserCacheDir
Return errors by UserConfigDir and UserCacheDir when XDG environment
variables contain relative paths.
Fixes #68470
Change-Id: Ib36b56d73b066e002023be55ecfe74d5c0eedb15
GitHub-Last-Rev:
c03f371a042c475a6f3f2259b06b140ade511404
GitHub-Pull-Request: golang/go#68471
Reviewed-on: https://go-review.googlesource.com/c/go/+/598655
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>