Ubuntu 18.04 Actions runner image is deprecated, use newer image (#2759)

* Ubuntu 18.04 Actions runner image is deprecated, use newer image

See https://github.blog/changelog/2022-08-09-github-actions-the-ubuntu-18-04-actions-runner-image-is-being-deprecated-and-will-be-removed-by-12-1-22/

* make it really use gcc 7

.github/workflows/build_cpp.yml

@@ -105,8 +105,8 @@ jobs:
     - name: Test BLAS bindings
       run: build_blas_bindings/dtest --runall -q
 
-  ubuntu-18_04-gcc-7:
+  ubuntu-20_04-gcc-7:
-    runs-on: 'ubuntu-18.04'
+    runs-on: 'ubuntu-20.04'
     steps:
     - uses: actions/checkout@v2
 
@@ -138,7 +138,10 @@ jobs:
         cd ..
 
     - name: Configure
-      run: cmake ${{ github.workspace }}/dlib/test -B build -DCMAKE_PREFIX_PATH=/home/runner/ffmpeg-n3.2.18_installation
+      run: |
+        export CC=/usr/bin/gcc-7
+        export CXX=/usr/bin/g++-7
+        cmake ${{ github.workspace }}/dlib/test -B build -DCMAKE_PREFIX_PATH=/home/runner/ffmpeg-n3.2.18_installation
 
     - name: Build just tests
       run: cmake --build build --config Release --target dtest --parallel 4