Populate Portage cache before running any emerge operations

Populating this cache in parallel with `egencache` goes way faster than
waiting for `emerge` to do so itself serially.

BUG=chromium:1017395
TEST=`./build_packages --board=grunt`

Cq-Depend: chromium:1876905, chromium:1877207
Change-Id: Ic767408fbf24e721c0461aea5a42a33baa340f4e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/1877287
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Chris McDonald <cjmcdonald@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
2 files changed