Add cache-provider: external to skip Gradle User Home caching (#1059)

Users relying on an external mechanism to save/restore Gradle User Home
(e.g. Develocity Artifact Cache) previously had to set cache-disabled:
true, which is confusing since caching isn't actually disabled — it's
just not managed by this action — and the Job Summary misleadingly
reported caching as "Disabled".

cache-provider: external skips Gradle User Home restore/save (same as
cache-disabled) but reports a distinct "External" status in the Job
Summary, explaining that caching is handled by another provider.

---------

Co-authored-by: Claude Sonnet 5 <[email protected]>

.github/workflows/demo-job-summary.yml

@@ -132,6 +132,23 @@ jobs:
       working-directory: .github/workflow-samples/kotlin-dsl
       run: ./gradlew assemble
 
+  cache-provider-external:
+    needs: build-distribution
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout sources
+      uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+    - name: Initialize integ-test
+      uses: ./.github/actions/init-integ-test
+
+    - name: Setup Gradle
+      uses: ./setup-gradle
+      with:
+        cache-provider: external
+    - name: Build kotlin-dsl project
+      working-directory: .github/workflow-samples/kotlin-dsl
+      run: ./gradlew assemble
+
   basic-caching:
     needs: build-distribution
     runs-on: ubuntu-latest