cmd/go: disable TestAccidentalGitCheckout for now

It seems it might not have ever worked.

Updates #22983

Change-Id: Icc022539aa2555486a65900abf97dfa30f92a1ea
Reviewed-on: https://go-review.googlesource.com/129615
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>

src/cmd/go/go_test.go

@@ -1293,6 +1293,7 @@ func TestGetGitDefaultBranch(t *testing.T) {
 
 func TestAccidentalGitCheckout(t *testing.T) {
 	testenv.MustHaveExternalNetwork(t)
+	testenv.SkipFlaky(t, 22983) // this test might not have ever worked; see issue.
 	if _, err := exec.LookPath("git"); err != nil {
 		t.Skip("skipping because git binary not found")
 	}