(CI) factor out shared setup into standalone action

.github/actions/setup-build-env/action.yml

@@ -0,0 +1,29 @@
+name: 'Setup Build Environment'
+description: 'Set up LLVM, MESA, and Flutter for builds'
+runs:
+  using: 'composite'
+  steps:
+    - uses: actions/checkout@v4
+    - name: Setup LLVM
+      uses: KyleMayes/install-llvm-action@v1
+      with:
+        version: "16"
+    - name: Add LLVM to PATH
+      shell: bash
+      run: |
+        echo "${{ github.workspace }}/llvm/bin" >> $GITHUB_PATH
+        echo "LD_LIBRARY_PATH=${{ github.workspace }}/llvm/lib/x86_64-unknown-linux-gnu/" >> $GITHUB_ENV
+    - name: Verify installation
+      shell: bash
+      run: clang --version
+    - name: Install MESA
+      shell: bash
+      run: |
+        sudo apt-get update
+        sudo apt-get install -y xvfb
+        sudo apt-get install -y libosmesa6-dev
+    - name: Set up Flutter
+      uses: subosito/flutter-action@v2
+      with:
+        channel: master
+        architecture: X64