cmd/go: don't run TestUserOverrideFlags in parallel

It creates files in the cmd/go directory, which can confuse other tests.

Fixes #22584.

Change-Id: Iad5a25c62e7d413af1648dbc5359ed78bfd61d2a
Reviewed-on: https://go-review.googlesource.com/76398Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/go_test.go

@@ -4342,7 +4342,8 @@ func TestUserOverrideFlags(t *testing.T) {
 
 	tg := testgo(t)
 	defer tg.cleanup()
-	tg.parallel()
+	// Don't call tg.parallel, as creating override.h and override.a may
+	// confuse other tests.
 	tg.tempFile("override.go", `package main
 
 import "C"