chore: pin flux-local to v5.3.1

.github/workflows/flux-diff.yaml

@@ -35,7 +35,7 @@ jobs:
           path: default
 
       - name: Diff Resources
-        uses: docker://ghcr.io/allenporter/flux-local:main
+        uses: docker://ghcr.io/allenporter/flux-local:v5.3.1
         with:
           args: >-
             diff ${{ matrix.resources }}