portage: Set PORTAGE_USERNAME when invoking setup_board

This variable is normally set by cros_sdk and is used to ensure that
src_unpack is ran by the same user that ran `repo sync`. Without this
git complains about using a repo owned by a different user.

BUG=b:394378820
TEST=BOARD=amd64-generic bazel build @portage//internal/sysroot:all

Change-Id: I1b45b374d505f4a048482aa68fda365003878c23
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/bazel/+/6250998
Auto-Submit: Raul Rangel <rrangel@chromium.org>
Tested-by: Raul Rangel <rrangel@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Raul Rangel <rrangel@chromium.org>
1 file changed