[autotest] update checksum in packager.process_all_packages

Original code has a bug that it does not update checksum for existing package.
This leads to packages being rebuild during build process, e.g.,
client-autotest.tar.bz2, has mismatched checksum in packages.checksum.

BUG=chromium:272500
TEST=run build_package in local chroot and verify checksum matches for
client-autotest.tar.bz2

Change-Id: I9db2496df3f99606e0eec313643a5051bde4f1eb
Reviewed-on: https://gerrit.chromium.org/gerrit/65911
Reviewed-by: Alex Miller <milleral@chromium.org>
Commit-Queue: Dan Shi <dshi@chromium.org>
Tested-by: Dan Shi <dshi@chromium.org>
1 file changed