build_system: manually install missing compilers

.github/workflows/build.yml

@@ -18,6 +18,15 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
+    - name: Install g++-7
+      if: ${{ matrix.compiler == 'g++-7' }}
+      run: sudo apt install g++-7
+    - name: Install g++-8
+      if: ${{ matrix.compiler == 'g++-8' }}
+      run: sudo apt install g++-8
+    - name: Install clang-8
+      if: ${{ matrix.compiler == 'clang++-8' }}
+      run: sudo apt install clang-8
     - name: Compile tests
       working-directory: build
       env:

.github/workflows/deploy.yml

@@ -20,8 +20,7 @@ jobs:
       working-directory: build
       env:
         PERSONAL_ACCESS_TOKEN: ${{ secrets.PERSONAL_ACCESS_TOKEN }}
-      run: |
-        git clone https://$GITHUB_ACTOR:$PERSONAL_ACCESS_TOKEN@github.com/$GITHUB_ACTOR/$GH_REPO.git
+      run: git clone https://$GITHUB_ACTOR:$PERSONAL_ACCESS_TOKEN@github.com/$GITHUB_ACTOR/$GH_REPO.git
     - name: Prepare formula
       working-directory: build
       run: |