Update CI to set compiler env vars conditionally

Adds conditional steps in the CI workflow to set CC and CXX environment variables only when matrix.cc is defined. Ensures builds use the correct compiler configuration based on the matrix setup.

Author: sean-parent

templates/.github/workflows/ci.yml.in

@@ -58,6 +58,17 @@ jobs:
           cmake --preset=default
           cmake --build --preset=default
           cmake --install build/default --prefix ${{ runner.temp }}/install
+        env:
+          CC: ${{ matrix.cc }}
+          CXX: ${{ matrix.cxx }}
+        if: ${{ matrix.cc }}
+
+      - name: Build and Install
+        run: |
+          cmake --preset=default
+          cmake --build --preset=default
+          cmake --install build/default --prefix ${{ runner.temp }}/install
+        if: ${{ !matrix.cc }}
 
       - name: Test find_package
         shell: bash