CI: Check out both the base and PR branches for "recent base" check

This should hopefully ensure that we end up with a single Git repository
that has both branches in it, enabling `git merge-base --is-ancestor` to
work correctly.

.github/workflows/checks.yml

@@ -12,10 +12,17 @@ jobs:
     name: Branch base is sufficiently recent
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - name: Check out the base branch
+        uses: actions/checkout@v3
         with:
           fetch-depth: 0
 
+      - name: Check out the PR branch
+        uses: actions/checkout@v3
+        with:
+          ref: ${{ github.head_ref }}
+          fetch-depth: 0
+
       - name: Ensure branch contains necessary commits for Tekton CI
         id: tekton
         # https://github.com/zcash/zcash/pull/6358