go/types,cmd/compile/internal/types2: unskip std and cmd in TestStdlib

CL 276272 accidentally skipped everything in TestStdlib while trying to
skip nested submodules of std and cmd.

For now, narrow the skip to just the problematic submodule rather than
trying to generalize. We can re-evaluate if it becomes a pattern to
vendor submodules in this way.

Fixes #46027

Change-Id: Ib355ff80dfbf17c3cf37d128a2f48d4216305267
Reviewed-on: https://go-review.googlesource.com/c/go/+/317869


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>