Remove all action sources: these have been migrated to 'gradle/actions'

Starting with the `v3` release, this action will delegate to `gradle/actions/setup-gradle`.
All repository sources have been migrated to https://github.com/gradle/actions, and
are no longer required in this repository.

Workflows testing the action functionality have been retained, but any workflows involving
building or inspecting code have also been migrated to `gradle/actions`.

.github/workflows/integ-test-restore-custom-gradle-home.yml

@@ -5,12 +5,8 @@ on:
     inputs:
       cache-key-prefix:
         type: string
-      download-dist:
-        type: boolean
-        default: false
 
 env:
-  DOWNLOAD_DIST: ${{ inputs.download-dist }}
   GRADLE_BUILD_ACTION_CACHE_KEY_PREFIX: restore-custom-gradle-home-${{ inputs.cache-key-prefix }}
   GRADLE_BUILD_ACTION_CACHE_DEBUG_ENABLED: true
 
@@ -24,8 +20,6 @@ jobs:
         echo "GRADLE_USER_HOME=$GITHUB_WORKSPACE/gradle-user-home" >> $GITHUB_ENV
     - name: Checkout sources
       uses: actions/checkout@v4
-    - name: Download distribution if required
-      uses: ./.github/actions/download-dist
     - name: Setup Gradle
       uses: ./
       with:
@@ -45,8 +39,6 @@ jobs:
         echo "GRADLE_USER_HOME=$GITHUB_WORKSPACE/gradle-user-home" >> $GITHUB_ENV
     - name: Checkout sources
       uses: actions/checkout@v4
-    - name: Download distribution if required
-      uses: ./.github/actions/download-dist
     - name: Setup Gradle
       uses: ./
       with:
@@ -66,8 +58,6 @@ jobs:
         echo "GRADLE_USER_HOME=$GITHUB_WORKSPACE/gradle-user-home" >> $GITHUB_ENV
     - name: Checkout sources
       uses: actions/checkout@v4
-    - name: Download distribution if required
-      uses: ./.github/actions/download-dist
     - name: Setup Gradle
       uses: ./
       with: