scriptworker's chain of trust verification expects a decision task's
MOBILE_HEAD_REPOSITORY to match the repository URL in metadata.source.
For pull requests that means we shouldn't set metadata.source to the
base repo.

This reverts commit bf984d2d.

This reverts commit f3a08e91.

Bug 1799696 - part 2: Change signing routes to support both APKs and components coming from the same repo

Co-authored-by: Christian Sadilek <christian.sadilek@gmail.com>