cmd/go: fix failure in TestScript/build_runtime_gcflags

This test case failed on the longtest builder. It relied on
runtime/internal/atomic not being compiled with the -l flag in the
cache. The test case now creates its own GOCACHE, similar to
build_cache_compile and a few others.

Also, mention the correct issue the test case verifies.

Fixes #29395

Change-Id: Id50e9dfc50db03fb11582d3dd6b69c3e1ed750eb
Reviewed-on: https://go-review.googlesource.com/c/156237


Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>