Test both rc and main branches daily

.github/workflows/daily.yml

@@ -8,7 +8,32 @@ on:
     - cron: '46 5 * * *'
 
 jobs:
+  # I'm sure there's a more lightweight way to do this, but this works well
+  # enough...
+  decide-branches:
+    name: Decide branches to test
+    outputs:
+      branches: ${{ steps.branches.outputs.branches }}
+    runs-on: ubuntu-latest
+    steps:
+      # Produce a JSON array of the branches that both exist and which I want
+      # to build regularly if they do exist.
+      - name: Report branches to build
+        id: branches
+        run: |
+          git ls-remote --heads "$GITHUB_SERVER_URL/$GITHUB_REPOSITORY" |
+            cut -f2 |
+            grep -e '^refs/heads/main$' -e '^refs/heads/rc$' |
+            jq -Rrs 'rtrimstr("\n") | split("\n") | @json "branches=\(.)"' |
+            tee /dev/stderr >> "$GITHUB_OUTPUT"
+
   build-test:
     uses: cygporter/workflows/.github/workflows/build-test.yml@v2
+    needs: decide-branches
+    strategy:
+      matrix:
+        branch: ${{ fromJSON(needs.decide-branches.outputs.branches) }}
+      fail-fast: false
     with:
       cygport_file: git.cygport
+      ref: ${{ matrix.branch }}