build_packages.sh: Migrate rebuild pkgs to Python

Get and parse force local build packages passed as envvars from the
Python script.

BUG=b:218521900
TEST=CQ

Cq-Depend: chromium:3635001
Change-Id: Ia4ede9b97c68f459f7469f052f94b068b2bedb76
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/3635758
Reviewed-by: Ram Chandrasekar <rchandrasekar@google.com>
Tested-by: Cindy Lin <xcl@google.com>
Commit-Queue: Cindy Lin <xcl@google.com>
1 file changed