Skip a GC test known to be flaky:

  https://issues.guix.gnu.org/issue/37425
  https://github.com/golang/go/issues/27636

--- go/src/runtime/gc_test.go	2019-09-16 23:10:18.200680387 +0200
+++ go/src/runtime/gc_test.go	2019-09-16 23:11:50.324360646 +0200
@@ -27,6 +27,9 @@ func TestGcSys(t *testing.T) {
 	if runtime.GOOS == "linux" && runtime.GOARCH == "arm64" {
 		t.Skip("skipping test; GOOS=linux GOARCH=arm64 https://github.com/golang/go/issues/27636")
 	}
+	if runtime.GOOS == "linux" {
+		t.Skip("skipping test; GOOS=linux https://issues.guix.gnu.org/issue/37425")
+	}
 	got := runTestProg(t, "testprog", "GCSys")
 	want := "OK\n"
 	if got != want {
