cmd/go: skip TestLinkerTmpDirIsDeleted if cgo is not enabled

Fixes builders that do not have cgo installed.

Change-Id: I719b7959226b0e67c3ffc11e071784787cabc5ab
Reviewed-on: https://go-review.googlesource.com/114235
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/go_test.go

@@ -6247,6 +6247,10 @@ func TestNoRelativeTmpdir(t *testing.T) {
 
 // Issue 24704.
 func TestLinkerTmpDirIsDeleted(t *testing.T) {
+	if !canCgo {
+		t.Skip("skipping because cgo not enabled")
+	}
+
 	tg := testgo(t)
 	defer tg.cleanup()
 	tg.parallel()