dashboard: test with -mmacosx-version-min set on darwin-amd64-10_15

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>

dashboard/builders.go

@@ -2307,6 +2307,13 @@ func init() {
 		HostType:       "host-darwin-10_15",
 		distTestAdjust: macTestPolicy,
 		buildsRepo:     defaultPlusExpBuild,
+		env: []string{
+			// Find out via at least one builder if having -mmacosx-version-min
+			// set via CGO_CFLAGS causes unexpected problems. This env var will
+			// also be set by cmd/release during a release. See issue 50892.
+			"CGO_CFLAGS=-mmacosx-version-min=10.13",
+		},
+		Notes: "Unlike other darwin/amd64 builders, this one sets CGO_CFLAGS=-mmacosx-version-min=10.13 in env.",
 	})
 	addBuilder(BuildConfig{
 		Name:           "darwin-amd64-11_0",