Bryan and I discussed that it might be a good idea to move the logic
that sets -mmacosx-version-min=nnn via CGO_CFLAGS on GOOS=darwin from
x/build/cmd/release to the cmd/go command. That way, if a problem
happens, it's reported not only during tests that the release script
runs, but also during normal pre/post-submit builders, thus sooner.

Until that happens (and in case it doesn't), set this environment
variable on at least one of our darwin/amd64 builders to improve
our test coverage for this edge case. This is inexpensive to do
compared to adding an entire new builder, and can be done quickly.

This is one of minor differences in environment between the tests
that run during by builders and during the release, and the long
term plan to catch all of them is still via nightly releases.

For golang/go#50892.
Updates golang/go#29205.

Change-Id: I5dd9a2f1dd457f54db6590d4ee181f81df8f7c38
Reviewed-on: https://go-review.googlesource.com/c/build/+/382055
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

For #50892

Change-Id: I14ff1c43b39687a0ba5e668ee962cecfb49e4beb
Reviewed-on: https://go-review.googlesource.com/c/go/+/381836
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

GoTool was added in CL 20967, and revised in CL 21292, for #14901.

I don't fully understand what problem the GoTool function was added to
solve: the discussion on that issue was pretty sparse, but it seems
like when we run tests of GOROOT packages they always know their own
location relative to GOROOT (and thus always know where to find the
'go' tool).

Lacking that understanding, I don't want to change its behavior, but I
do at least want to verify that it resolves to the real 'go' tool in
the common case (running 'go test' on a package in GOROOT/src).

For #50892
For #50893
Updates #14901

Change-Id: I06d831e6765be631dfc4854d7fddc3d27fc1de34
Reviewed-on: https://go-review.googlesource.com/c/go/+/381834
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>