cmd/go: allow the user to specify ar via an environment variable
This allows one to customize which ar to use by fetching its path
from the environment. This way one can swap it out for a
different implementation.
Change-Id: I40d8cbd8a69e97b5254e66081d9bf0b726c10366
GitHub-Last-Rev:
4aa1d631eaca58be6c3a4be40f7404fa75a0db25
GitHub-Pull-Request: golang/go#28746
Reviewed-on: https://go-review.googlesource.com/c/149117
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>