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

Nest actions/gradle-actions@v5 rejects cache-provider; v6 supports it
(and defaults to enhanced — keep Gitea on external). Cache ~/apt-cache
so xvfb debs can skip apt-get update when Actions cache is available.

.github/workflows/gradle.yml

@@ -21,37 +21,63 @@ jobs:
   build:
 
     runs-on: ubuntu-latest
+    timeout-minutes: 90
 
     steps:
     - uses: actions/checkout@v6
+
+    # Same pattern as TDLM ForestryMC/IndustrialCraft2 setup-build: cache the JDK toolchain.
     - name: Set up JDK 17
       uses: actions/setup-java@v6
       with:
         distribution: 'liberica'
         java-version: '17'
         java-package: 'jdk+fx'
+        cache-jdk: true
+
+    # 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
-      # Nest hosts this under actions/gradle-actions (mirror of github.com/gradle/actions).
-      # uses: gradle/actions/... makes act clone git.nest…/gradle/actions which is not seeded.
-      uses: actions/gradle-actions/setup-gradle@v5
+      uses: actions/gradle-actions/setup-gradle@v6
       with:
         cache-cleanup: never
-        # Only main writes the dependency cache; pull requests read it.
-        cache-read-only: ${{ github.ref != 'refs/heads/main' }}
-    # The repository's wrapper downloads Gradle itself, so cache that download too.
-    - name: Cache Gradle distribution
+        cache-provider: ${{ github.server_url == 'https://github.com' && 'basic' || 'external' }}
+
+    # Cache wrapper + dependency/build caches (covers what setup-gradle skips under external).
+    - name: Cache Gradle home
       uses: actions/cache@v4
       with:
         path: |
+          ~/.gradle/caches
           ~/.gradle/wrapper
           .gradle/wrapper-neo
-        key: gradle-dist-${{ hashFiles('gradle/wrapper/gradle-wrapper.properties', 'gradle/wrapper/GradleWrapperNeo.java') }}
+        key: gradle-home-${{ runner.os }}-${{ hashFiles('gradle/wrapper/gradle-wrapper.properties', 'gradle/wrapper/GradleWrapperNeo.java', '**/libs.versions.toml', '**/*.gradle.kts', '**/*.gradle') }}
+        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.
     - name: Build and check with Gradle
@@ -60,26 +86,31 @@ jobs:
         JAVA_TOOL_OPTIONS: -Djava.awt.headless=true
         MICROSOFT_AUTH_ID: ${{ secrets.MICROSOFT_AUTH_ID }}
         CURSEFORGE_API_KEY: ${{ secrets.CURSEFORGE_API_KEY }}
+
     - name: Get short SHA
       run: echo "SHORT_SHA=${GITHUB_SHA::7}" >> $GITHUB_ENV
+
     - name: Upload JAR
       uses: actions/upload-artifact@v7
       with:
         name: HMCL-${{ env.SHORT_SHA }}-jar
         path: HMCL/build/libs/HMCL-*.jar
         archive: false
+
     - name: Upload EXE
       uses: actions/upload-artifact@v7
       with:
         name: HMCL-${{ env.SHORT_SHA }}-exe
         path: HMCL/build/libs/HMCL-*.exe
         archive: false
+
     - name: Upload SH
       uses: actions/upload-artifact@v7
       with:
         name: HMCL-${{ env.SHORT_SHA }}-sh
         path: HMCL/build/libs/HMCL-*.sh
         archive: false
+
     - name: Upload DEB
       uses: actions/upload-artifact@v7
       with: