x/build: add a darwin longtest builder

[bcmills]

Now that we have a Windows longtest builder (#26529), I'm wondering where else we might have a coverage gap.

It would be nice to have a darwin builder running non-short tests too. I doubt that it will turn up many regressions for cmd/go in particular, but the system calls are enough different — and the installed base of cmd/go users on macOS is large enough — that I suspect it would have a net positive value.

CC @dmitshur @cagedmantis @toothrot @bradfitz

[bcmills]

(This message brought to you by cmd/go.tooSlow, which probably ought to just Not Be a Thing.)

go/src/cmd/go/go_test.go

Lines 49 to 53 in a23f9af

	// In -short mode; skip test, except run it on the {darwin,linux,windows}/amd64 builders.
	if testenv.Builder() != "" && runtime.GOARCH == "amd64" && (runtime.GOOS == "linux" || runtime.GOOS == "darwin" || runtime.GOOS == "windows") {
	return
	}
	t.Skip("skipping test in -short mode")