Check if *-MAlonzo branch exists on each run

.github/workflows/build.yml

@@ -17,22 +17,29 @@ jobs:
   MAlonzo:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - name: Check if ${{ env.MAlonzo_branch }} exists
+        uses: actions/checkout@v4
         with:
-          ref: MAlonzo-code
+          ref: ${{ env.MAlonzo_branch }}
+        id: MAlonzo-exists
+        continue-on-error: true
+        if: github.ref != 'refs/heads/master'
       - name: Create branch ${{ env.MAlonzo_branch }} for generated code
-        if: github.event_name == 'pull_request' && github.event.action == 'opened'
+        uses: actions/checkout@v4
+        with:
+          ref: MAlonzo-code
+        if: github.ref != 'refs/heads/master' && steps.MAlonzo-exists.outcome == 'failure'
         run: |
           git checkout -b ${{ env.MAlonzo_branch }} origin/MAlonzo-code
           git push origin ${{ env.MAlonzo_branch }}
-      - uses: actions/checkout@v4
       - uses: cachix/install-nix-action@v20
         with:
           nix_path: nixpkgs=channel:nixos-unstable
           extra_nix_config: |
             trusted-public-keys = hydra.iohk.io:f/Ea+s+dFdN+3Y/G+FDgSq+a5NEWhJGzdjvKNGv0/EQ= cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY=
             substituters = https://cache.iog.io https://cache.nixos.org/
       - name: Commit generated code at ${{ env.MAlonzo_branch }}
+        uses: actions/checkout@v4
         if: github.ref != 'refs/heads/master'
         run: |
             nix-build -A ledger.hsSrc -j1 -o outputs/MAlonzo