cmd/go: close unintentionally left open file

cmd/go: close unintentionally left open file

cmd/go/internal/work.Builder.updateBuildID left a file opened.
But opened files cannot be deleted on Windows, so cmd/go just
leaves these files in %TMP% directory.

Close the file so deletion can succeed.

Fixes #22650

Change-Id: Ia3ea62f6ec7208d73972eae2e17fb4a766407914
Reviewed-on: https://go-review.googlesource.com/76810
Reviewed-by: Dave Cheney <dave@cheney.net>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>