add reusable strings

.github/workflows/tests.yml

@@ -11,6 +11,13 @@ jobs:
         runs-on: ubuntu-latest
         steps:
         - uses: actions/checkout@v4
+        - name: Set reusable strings
+          # Turn repeated input strings (such as the build output directory) into step outputs. These step outputs can be used throughout the workflow file.
+          id: strings
+          shell: bash
+          run: |
+            echo "build-output-dir=${{ github.workspace }}/build" >> "$GITHUB_OUTPUT"
+  
         - name: Build ADOL-C
           run: >  
             cmake -B ${{ steps.strings.outputs.build-output-dir }}