os: use AddCleanup to close files

os: use AddCleanup to close files

This changes the finalizer mechanism used to close files from
runtime.SetFinalizer to runtime.AddCleanup.

Updates #70907

Change-Id: I47582b81b0ed69609dd9dac158ec7bb8819c8c77
Reviewed-on: https://go-review.googlesource.com/c/go/+/638555
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>