Apparently checkout before using a local workflow

.github/workflows/main_matrix.yml

@@ -97,6 +97,13 @@ jobs:
 
     runs-on: ubuntu-latest
     steps:
+      - name: Checkout code
+          uses: actions/checkout@v3
+          with:
+            submodules: "recursive"
+            ref: ${{github.event.pull_request.head.ref}}
+            repository: ${{github.event.pull_request.head.repo.full_name}}
+
       - uses: ./.github/workflows/build_esp32.yml
         with:
           board: ${{ matrix.board }}