ci: bump setup-gradle to v6 and cache apt xvfb debs

Match the ci-cache-optimize PR: v6 understands cache-provider; cache
apt debs for xvfb alongside Gradle home.

.github/workflows/gradle.yml

@@ -35,11 +35,11 @@ jobs:
         java-package: 'jdk+fx'
         cache-jdk: true
 
-    # Nest hosts github.com/gradle/actions as actions/gradle-actions.
-    # On Gitea, setup-gradle's default GitHub Actions cache hits ECONNREFUSED; org repos
-    # (ForestryMC/IC2) use cache-provider external off github.com and cache Gradle home below.
+    # Nest mirrors github.com/gradle/actions as actions/gradle-actions (also as gradle/actions).
+    # v5 rejects cache-provider; v6 supports it. Default on v6 is "enhanced" (commercial) —
+    # on Gitea use external and cache Gradle home with actions/cache below (org IC2/Forestry pattern).
     - name: Setup Gradle
-      uses: actions/gradle-actions/setup-gradle@v5
+      uses: actions/gradle-actions/setup-gradle@v6
       with:
         cache-cleanup: never
         cache-provider: ${{ github.server_url == 'https://github.com' && 'basic' || 'external' }}
@@ -56,12 +56,27 @@ jobs:
         restore-keys: |
           gradle-home-${{ runner.os }}-
 
+    # xvfb apt debs — avoids apt-get update+download on every cold container when Actions cache works.
+    - name: Cache apt packages
+      uses: actions/cache@v4
+      with:
+        path: ~/apt-cache
+        key: apt-xvfb-${{ runner.os }}-${{ runner.arch }}
+
     - name: Install Xvfb
       run: |
-        # Installing is the slowest setup step, so skip it when the runner already has xvfb, and
-        # only refresh the package index when a plain install fails.
-        command -v xvfb-run >/dev/null || sudo apt-get install -y --no-install-recommends xvfb \
+        if command -v xvfb-run >/dev/null; then
+          exit 0
+        fi
+        sudo mkdir -p /var/cache/apt/archives/partial
+        if [ -d "${HOME}/apt-cache" ] && [ -n "$(ls -A "${HOME}/apt-cache" 2>/dev/null)" ]; then
+          sudo cp -a "${HOME}/apt-cache/." /var/cache/apt/archives/
+        fi
+        # Prefer offline/cached debs first; only refresh the index when that fails.
+        sudo apt-get install -y --no-install-recommends xvfb \
           || { sudo apt-get update && sudo apt-get install -y --no-install-recommends xvfb; }
+        mkdir -p "${HOME}/apt-cache"
+        cp -a /var/cache/apt/archives/*.deb "${HOME}/apt-cache/" 2>/dev/null || true
 
     # One Gradle run builds, tests and runs the code and translation checks, so the
     # project is configured and compiled once instead of in two workflows.

.github/workflows/publish-tdlm.yml

@@ -55,7 +55,7 @@ jobs:
           cache-jdk: true
 
       - name: Setup Gradle
-        uses: actions/gradle-actions/setup-gradle@v5
+        uses: actions/gradle-actions/setup-gradle@v6
         with:
           cache-cleanup: never
           cache-provider: ${{ github.server_url == 'https://github.com' && 'basic' || 'external' }}
@@ -71,10 +71,25 @@ jobs:
           restore-keys: |
             gradle-home-${{ runner.os }}-
 
+      - name: Cache apt packages
+        uses: actions/cache@v4
+        with:
+          path: ~/apt-cache
+          key: apt-xvfb-${{ runner.os }}-${{ runner.arch }}
+
       - name: Install Xvfb
         run: |
-          command -v xvfb-run >/dev/null || sudo apt-get install -y --no-install-recommends xvfb \
+          if command -v xvfb-run >/dev/null; then
+            exit 0
+          fi
+          sudo mkdir -p /var/cache/apt/archives/partial
+          if [ -d "${HOME}/apt-cache" ] && [ -n "$(ls -A "${HOME}/apt-cache" 2>/dev/null)" ]; then
+            sudo cp -a "${HOME}/apt-cache/." /var/cache/apt/archives/
+          fi
+          sudo apt-get install -y --no-install-recommends xvfb \
             || { sudo apt-get update && sudo apt-get install -y --no-install-recommends xvfb; }
+          mkdir -p "${HOME}/apt-cache"
+          cp -a /var/cache/apt/archives/*.deb "${HOME}/apt-cache/" 2>/dev/null || true
 
       - name: Build executables
         run: xvfb-run --auto-servernum ./gradlew :HMCL:makeExecutables --no-daemon --parallel