Fix coverage github action by using clang++ without a specific version (#769)

Github fails to find clang++-10.
Since we install llvm without a specific version number, we shouldn't
use clang++-10 with a version number.

I tried installing llvm-10, but that package is not available on github
(any more?).

.github/workflows/build.yml

@@ -89,7 +89,7 @@ jobs:
         sudo apt install llvm
     - name: make coverage
       run: |
-        CXX=clang++-10 make -j2 config=coverage native=1 coverage
+        CXX=clang++ make -j2 config=coverage native=1 coverage
     - name: upload coverage
       uses: codecov/codecov-action@v3
       with: