[ci] Use generic syntax for var expansion

Author: ricab

.github/workflows/test.yml

@@ -88,12 +88,12 @@ jobs:
       run: echo "CXX=${{matrix.compiler}}" >> $GITHUB_ENV
 
     - name: Configure CMake
-      run: cmake -Wdev -Werror=dev -B ${{github.workspace}}/build -DCMAKE_BUILD_TYPE=${BUILD_TYPE}
+      run: cmake -Wdev -Werror=dev -B ${{github.workspace}}/build -DCMAKE_BUILD_TYPE=${{env.BUILD_TYPE}}
           ${{matrix.extra_build_flags}}
 
     - name: Build
       working-directory: ${{github.workspace}}/build
-      run: cmake --build . --config ${BUILD_TYPE} --parallel ${num_cpus}
+      run: cmake --build . --config ${{env.BUILD_TYPE}} --parallel ${{env.num_cpus}}
 
     - name: Test
       working-directory: ${{github.workspace}}/build