Fix CI runs on develop.

This can't be easily verified until it's merged, however I have high
confidence that this should fix it as we've done similar fallback syntax
for env variables in the past (e.g. '||') and 'origin/develop' was the
value previously used for the base commit when computing affected tests.

.github/workflows/unit_tests.yml

@@ -76,8 +76,8 @@ jobs:
         id: compute-test-matrix
         env:
           compute_all_targets: ${{ contains(github.event.pull_request.title, '[RunAllTests]') }}
-          # See: https://docs.github.com/en/webhooks-and-events/webhooks/webhook-events-and-payloads#pull_request.
-          base_commit_hash: ${{ github.event.pull_request.base.sha }}
+          # See: https://docs.github.com/en/webhooks-and-events/webhooks/webhook-events-and-payloads#pull_request. Defer to origin/develop outside a PR (such as develop's main CI runs).
+          base_commit_hash: ${{ github.event.pull_request.base.sha || "origin/develop" }}
         # https://unix.stackexchange.com/a/338124 for reference on creating a JSON-friendly
         # comma-separated list of test targets for the matrix.
         run: |