Bump setup-dune to v2.0.0+patch-4 and add cache-readonly across repos

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: mbarbin

.github/workflows/ci.yml

@@ -36,11 +36,12 @@ jobs:
           echo "DUNE_WORKSPACE=$PWD/dune-workspace-${{ matrix.ocaml-version }}" >> "$GITHUB_ENV"
 
       - name: Setup Dune
-        uses: mbarbin/setup-dune@d38f7c80df59974cbe417c7f2a849219d54b4fdf # v2.0.0+patch-3
+        uses: mbarbin/setup-dune@ae2985bcdedc6fb24e5743d6f9dace9cf262b41c # v2.0.0+patch-4
         with:
           version: "3.21.0"
           workspace: ${{ env.DUNE_WORKSPACE }}
           cache-prefix: "main-ci-${{ matrix.ocaml-version }}"
+          cache-readonly: ${{ github.ref != 'refs/heads/main' }}
           steps: install-dune enable-pkg lazy-update-depexts install-gpatch install-depexts
 
       - name: Build and Run tests

.github/workflows/deploy-odoc.yml

@@ -30,11 +30,12 @@ jobs:
           echo "DUNE_WORKSPACE=$PWD/dune-workspace-${{ env.OCAML_VERSION }}" >> "$GITHUB_ENV"
 
       - name: Setup Dune
-        uses: mbarbin/setup-dune@d38f7c80df59974cbe417c7f2a849219d54b4fdf # v2.0.0+patch-3
+        uses: mbarbin/setup-dune@ae2985bcdedc6fb24e5743d6f9dace9cf262b41c # v2.0.0+patch-4
         with:
           version: "3.21.0"
           workspace: ${{ env.DUNE_WORKSPACE }}
-          cache-prefix: "deploy-odoc-${{ env.OCAML_VERSION }}"
+          cache-prefix: "main-ci-${{ env.OCAML_VERSION }}"
+          cache-readonly: true
           steps: install-dune enable-pkg lazy-update-depexts install-gpatch install-depexts
 
       - name: Build Odoc Pages

.github/workflows/dune-pkg-more-ci.yml

@@ -49,11 +49,12 @@ jobs:
           echo "PACKAGES=auto-format,auto-format-tests,auto-format-example" >> "$GITHUB_ENV"
 
       - name: Setup Dune
-        uses: mbarbin/setup-dune@d38f7c80df59974cbe417c7f2a849219d54b4fdf # v2.0.0+patch-3
+        uses: mbarbin/setup-dune@ae2985bcdedc6fb24e5743d6f9dace9cf262b41c # v2.0.0+patch-4
         with:
           version: "3.21.0"
           workspace: ${{ env.DUNE_WORKSPACE }}
           cache-prefix: ${{ matrix.ocaml-version }}
+          cache-readonly: ${{ github.ref != 'refs/heads/main' }}
           only-packages: ${{ env.PACKAGES }}
           steps: install-dune enable-pkg lazy-update-depexts install-gpatch install-depexts