cmd/go: run the 'go build' command in TestACL in the temp directory

Otherwise, when the 'cmd' module is added the test will run as if in module 'cmd'.

While we're here, remove an unnecessary os.Chdir in TestAbsolutePath:
we can instead set the Dir on the 'go build' command instead. Then we
can run the tests in this file in parallel with everything else.

Updates #30228

Change-Id: I13ecd7ec93bc1041010daec14d76bac10e0c89be
Reviewed-on: https://go-review.googlesource.com/c/go/+/165744


Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>