portage: Switch sdk_to_archive over to using `mount()` This cleans up a workaround where we were ignoring some files and directories that were created by the container set up. BUG=b:308661552 TEST=BOARD=brya bazel build @portage//internal/sdk/stage2:tarball Change-Id: Ib6a1a4f38a1134fc2fce95efcd046619bd59282e Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/bazel/+/5532401 Tested-by: Raul Rangel <[email protected]> Reviewed-by: Shuhei Takahashi <[email protected]> Commit-Queue: Raul Rangel <[email protected]>