Set matching CC alongside CXX in Linux CI matrix

The Configure step only set CXX, so CMake's C compiler detection
fell back to whatever the default happened to be on the runner,
independent of which C++ compiler the matrix entry was actually
testing (e.g. CXX=clang++-3.6 but CC left to detect GCC 11).

Derive CC from the same matrix.cxx value the job already installs
a matching compiler for.

.github/workflows/linux.yml

@@ -174,6 +174,13 @@ jobs:
         sudo apt install locales-all
         cmake -E make_directory ${{runner.workspace}}/build
 
+    - name: Select matching C compiler
+      run: |
+        cc=${{matrix.cxx}}
+        cc=${cc/clang++/clang}
+        cc=${cc/g++/gcc}
+        echo "CC=$cc" >> "$GITHUB_ENV"
+
     - name: Configure
       working-directory: ${{runner.workspace}}/build
       env: