Fixed github actions (#18)

Use gfortran-12, which is available on all systems.

.github/workflows/test.yml

@@ -29,7 +29,7 @@ jobs:
     - name: cmake
       run: cmake . -DCMAKE_TOOLCHAIN_FILE=conan_toolchain.cmake -DCMAKE_BUILD_TYPE=${{ env.BUILD_TYPE }} -DCMAKE_VERBOSE_MAKEFILE:BOOL=ON
       env:
-        FC: gfortran-13
+        FC: gfortran-12
     - name: cmake build
       run: cmake --build . --config ${{ env.BUILD_TYPE }}
     - name: ctest