fix: use Ubuntu 20.04 container for x86_64 Linux builds

.github/workflows/release.yml

@@ -86,6 +86,7 @@ jobs:
           - target: x86_64-linux-gnu
             os: ubuntu-22.04
             runner: ubuntu-22.04
+            container: ubuntu:20.04
 
           # Linux ARM64 build
           - target: aarch64-linux-gnu
@@ -103,13 +104,29 @@ jobs:
             runner: macos-14
 
     runs-on: ${{ matrix.runner }}
+    container: ${{ matrix.container }}
 
     steps:
     - name: Checkout
       uses: actions/checkout@v4
 
+    - name: Install build dependencies (Linux containers)
+      if: matrix.container == 'ubuntu:20.04'
+      run: |
+        export DEBIAN_FRONTEND=noninteractive
+        apt-get update
+        apt-get install -y build-essential ninja-build git ca-certificates wget python3 python3-pip
+
+        # Install newer CMake (LLVM requires 3.20+, Ubuntu 20.04 has 3.16)
+        CMAKE_VERSION="3.25.3"
+        wget -O cmake.tar.gz "https://github.com/Kitware/CMake/releases/download/v${CMAKE_VERSION}/cmake-${CMAKE_VERSION}-linux-x86_64.tar.gz"
+        tar -xzf cmake.tar.gz -C /opt
+        ln -sf /opt/cmake-${CMAKE_VERSION}-linux-x86_64/bin/* /usr/local/bin/
+        rm cmake.tar.gz
+        cmake --version
+
     - name: Install build dependencies (Linux)
-      if: matrix.os == 'ubuntu-22.04' || matrix.os == 'ubuntu-22.04-arm'
+      if: matrix.os == 'ubuntu-22.04-arm'
       run: |
         sudo apt-get update
         sudo apt-get install -y build-essential cmake ninja-build