use ubuntu 22.04 for test old compilers

The Ubuntu 20.04 Actions runner image will begin deprecation on 2025-02-01 and will be fully unsupported by 2025-04-01 actions/runner-images#11101

.github/workflows/build.yml

@@ -35,7 +35,7 @@ env:
 jobs:
   ubuntu-low-compiler:
     if: ${{ !(github.event_name == 'push' && github.ref == 'refs/heads/master' && github.repository != 'elfmz/far2l') }}
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-22.04
 
     strategy:
       fail-fast: false
@@ -57,11 +57,6 @@ jobs:
       - name: Checkout source
         uses: actions/checkout@v4
 
-      - name: Install compilers
-        run: >
-          sudo apt-get update;
-          sudo apt-get -y install gcc-7 g++-7 clang-7
-
       - name: Install far2l dependencies
         run: >
          sudo apt-get -y install
@@ -70,6 +65,12 @@ jobs:
          libssl-dev libsmbclient-dev libnfs-dev libneon27-dev libssh-dev
          libarchive-dev
          python3-dev python3-cffi
+
+      - name: Install compilers
+        run: >
+          echo "deb [arch=amd64] http://archive.ubuntu.com/ubuntu focal main universe" | sudo tee /etc/apt/sources.list
+          sudo apt-get update;
+          sudo apt-get -y install gcc-7 g++-7 clang-7          
  
       - name: Create Build Environment
         # Create a separate build directory as working directory for all subsequent commands