fixup! CI: add a target with older Ubuntu to test older compilers

Author: robUx4

.github/workflows/ubuntu-20.04.yaml

@@ -6,16 +6,57 @@ on:
     # branches: [ master ]
 
 jobs:
-  Oldest:
+  GCC-7:
     runs-on: ubuntu-20.04
-    strategy:
-      matrix:
-        cxx: ['g++', 'clang++-10']
     env:
       CMAKE_OPTIONS: -DDEV_MODE=ON -DBUILD_TESTING=ON
       CMAKE_MATROSKA_OPTIONS: -DBUILD_EXAMPLES=ON
-      CXX: ${{matrix.cxx}}
     steps:
+      - uses: egor-tensin/setup-gcc@v1
+        with:
+          version: 7
+
+      - name: list compilers
+        run: dpkg --list | grep compiler
+
+      - name: Get pushed code
+        uses: actions/checkout@v4
+
+      - name: Checkout libebml
+        uses: actions/checkout@v4
+        with:
+          repository: Matroska-Org/libebml
+          path: libebml
+          # minimum version we support ref: 'release-1.4.3'
+
+      - name: Configure libebml
+        run: cmake -S libebml -B libebml/_build ${{ env.CMAKE_OPTIONS }}
+
+      - name: Build libebml
+        run: cmake --build libebml/_build --parallel
+
+      - name: Install libebml
+        run: cmake --install libebml/_build --prefix ${GITHUB_WORKSPACE}/_built
+
+      - name: Configure
+        run: cmake -S . -B _build  ${{ env.CMAKE_OPTIONS }} ${{ env.CMAKE_MATROSKA_OPTIONS }} -DEBML_DIR="${GITHUB_WORKSPACE}/_built/lib/cmake/EBML"
+
+      - name: Build
+        run: cmake --build _build --parallel
+
+      - name: Test installation
+        run: cmake --install _build --prefix ${GITHUB_WORKSPACE}/_built
+
+  Clang-7:
+    runs-on: ubuntu-20.04
+    env:
+      CMAKE_OPTIONS: -DDEV_MODE=ON -DBUILD_TESTING=ON
+      CMAKE_MATROSKA_OPTIONS: -DBUILD_EXAMPLES=ON
+    steps:
+      - uses: egor-tensin/setup-clang@v1
+        with:
+          version: 7
+
       - name: list compilers
         run: dpkg --list | grep compiler