Move the code handling CROS_ARTIFACTS_TMP_DIR

The previous iteration of this code didn't work when emerge
was used -- only ebuild.

BUG=b:187795688
TEST=CQ, local testing

Change-Id: Iaf34cfbeeb2df43bebfa6a29b7ef58d6bb754f32
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/portage_tool/+/3695954
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Michael Benfield <mbenfield@google.com>
Reviewed-by: George Burgess <gbiv@chromium.org>
Commit-Queue: Michael Benfield <mbenfield@google.com>
2 files changed