fix noisy update license PR

[elrayle]

Description

A cron job runs nightly to keep the list of license up to date. It is supposed to exit early if there haven't been any changes since the last update of the PR it manages. But once there is a change to license files, it always registers a change.

Expected Behavior

• cron runs 1st night and there are no new licenses -- workflow exits early
• cron runs 2nd night and there are 2 new licenses -- creates the branch, makes a commit with the new licenses, and creates a PR
• cron runs 3rd night and there are no new licenses -- workflow exits early
• cron runs 4th night and there is 1 new license -- creates a commit to the existing branch with the new license and updates the existing PR

Actual Behavior

• cron runs 1st night and there are no new licenses -- workflow exits early (CORRECT)
• cron runs 2nd night and there are 2 new licenses -- creates the branch, makes a commit with the new licenses, and creates a PR (CORRECT)
• cron runs 3rd night and there are no new licenses -- updates the existing PR (WRONG)
• cron runs 4th night and there is 1 new license -- creates a commit to the existing branch with the new license and updates the existing PR (CORRECT)

The PR updates on the 3rd night because the source licenses are being compared against the main branch instead of the PR's branch. This makes the license files appear to be different even though there are no new changes on the 3rd night.