Use trimja-action

Use trimja ourselves to reduce the amount we need to build on pull
requests.  Unfortunately the Ninja file generated from CMake doesn't
have good dependencies for the tests.  The tests don't have a proper
dependency on the `trimja` executable, nor the test files that it reads.

For the moment we pass both `trimja` and `test` into `affected` so that
everything is always built.  However, we should look into improving
this.

.github/workflows/ci.yaml

@@ -24,25 +24,32 @@ jobs:
     steps:
     - uses: actions/checkout@v4
     - uses: seanmiddleditch/gha-setup-ninja@master
-    - name: Set reusable strings
-      id: strings
-      shell: bash
-      run: |
-        echo "build-output-dir=${{ github.workspace }}/build" >> "$GITHUB_OUTPUT"
     - name: Configure CMake
       run: >
-        cmake -B ${{ steps.strings.outputs.build-output-dir }}
+        cmake -B output
         -DCMAKE_CXX_COMPILER=${{ matrix.environment.cpp }}
         -DCMAKE_C_COMPILER=${{ matrix.environment.c }}
         ${{ format('-D{0}={1}', matrix.environment.name == 'Windows' && 'CMAKE_CONFIGURATION_TYPES' || 'CMAKE_BUILD_TYPE', matrix.build_type) }}
         ${{ matrix.environment.name != 'Windows' && '-G Ninja' || ''}}
+    - uses: elliotgoodrich/trimja-action@v1
+      # Skip Windows (which doesn't yet use Ninja) and releases
+      if: matrix.environment.name != 'Windows' && !startsWith(github.ref, 'refs/tags/')
+      with:
+        path: output/build.ninja
+        build-configuration: ${{ matrix.build_type }}
+        # CTest does not have the right dependencies so we always build everything, we
+        # won't gain any time, but will verify that trimja can at least handle a Ninja
+        # file generated by CMake.
+        affected: |
+          test
+          trimja
     - name: Build
-      run: cmake --build ${{ steps.strings.outputs.build-output-dir }} --config ${{ matrix.build_type }}
+      run: cmake --build output --config ${{ matrix.build_type }}
     - name: Test
-      run: ctest --test-dir ${{ steps.strings.outputs.build-output-dir }} --build-config ${{ matrix.build_type }} --output-on-failure
+      run: ctest --test-dir output --build-config ${{ matrix.build_type }} --output-on-failure
     - name: Install
       if: startsWith(github.ref, 'refs/tags/') && matrix.build_type == 'Release'
-      run: cmake --build ${{ steps.strings.outputs.build-output-dir }} --config ${{ matrix.build_type }} --target package
+      run: cmake --build output --config ${{ matrix.build_type }} --target package
     - name: Generate Changelog
       run: git show -s --format='%b' > build/CHANGELOG.txt
       if: startsWith(github.ref, 'refs/tags/') && matrix.build_type == 'Release'