Fix simplified GitHub file

Tested in https://github.com/EmilyBourne/gyselalibxx/

See merge request gysela-developpers/gyselalibxx!420

--------------------------------------------

.github/workflows/tests.yml

@@ -92,23 +92,27 @@ jobs:
     steps:
       - name: Checkout code
         uses: actions/checkout@v3
+        with:
+          path: code_branch
       - name: Checkout documentation
         uses: actions/checkout@v3
         with:
           ref: documentation
           path: doc_branch
+      - name: Install doxygen
+        run: |
+          sudo apt install doxygen
+        shell: bash
       - name: Generate docs
         run: |
-          mkdir build
-          cd build
-          cmake -DGYSELALIBXX_COMPILE_SOURCE=OFF -DBUILD_DOCUMENTATION=1 ../code_branch
-          make doc
+          cmake -DGYSELALIBXX_COMPILE_SOURCE=OFF -DBUILD_DOCUMENTATION=1 -B build-docs code_branch
+          cmake --build build-docs
         shell: bash
       - name: Update documentation
         run: |
           cd doc_branch
           git rm -r *
-          cp -r ../build/docs/html/* .
+          cp -r ../build-docs/docs/html/* .
           git add .
           git config --global user.name "GitHub CI Documentation builder"
           git config --global user.email "[email protected]"