Fix HEAD^1 resolution: fetch-depth 2 on check-addon-changes checkout

Confirmed on the live run: a depth-1 (default) shallow checkout of the
merge commit truncates parent refs at that boundary commit entirely, so
git rev-parse HEAD^1 fails with "unknown revision" even though the merge
commit itself is checked out fine. Bumping this job's checkout to
fetch-depth: 2 pulls in both the merge commit and its two parents, making
HEAD^1 resolvable with a real tree to diff against.

.github/workflows/onpr_check-pr.yaml

@@ -19,6 +19,10 @@ jobs:
     steps:
       - name: Checkout repo
         uses: actions/checkout@v7
+        with:
+          # Need the merge commit's parents resolvable (HEAD^1 below): a depth-1 shallow
+          # checkout truncates parent refs entirely at the boundary commit.
+          fetch-depth: 2
       - name: Find changed addon directories
         id: find_addons
         run: |