ci: Use nproc-2 to set parallelism for builds and tests (#5939)

This change reduces the number of cores used to build and test, as using all cores may be contributing to occasional build and test failures.

Co-authored-by: Bart Thomee <11445373+bthomee@users.noreply.github.com>

.github/workflows/publish-docs.yml

@@ -23,6 +23,7 @@ defaults:
 
 env:
   BUILD_DIR: .build
+  NPROC_SUBTRACT: 2
 
 jobs:
   publish:
@@ -33,6 +34,13 @@ jobs:
     steps:
       - name: Checkout repository
         uses: actions/checkout@08eba0b27e820071cde6df949e0beb9ba4906955 # v4.3.0
+
+      - name: Get number of processors
+        uses: XRPLF/actions/.github/actions/get-nproc@046b1620f6bfd6cd0985dc82c3df02786801fe0a
+        id: nproc
+        with:
+          subtract: ${{ env.NPROC_SUBTRACT }}
+
       - name: Check configuration
         run: |
           echo 'Checking path.'
@@ -46,12 +54,16 @@ jobs:
 
           echo 'Checking Doxygen version.'
           doxygen --version
+
       - name: Build documentation
+        env:
+          BUILD_NPROC: ${{ steps.nproc.outputs.nproc }}
         run: |
           mkdir -p "${BUILD_DIR}"
           cd "${BUILD_DIR}"
           cmake -Donly_docs=ON ..
-          cmake --build . --target docs --parallel $(nproc)
+          cmake --build . --target docs --parallel ${BUILD_NPROC}
+
       - name: Publish documentation
         if: ${{ github.ref_type == 'branch' && github.ref_name == github.event.repository.default_branch }}
         uses: peaceiris/actions-gh-pages@4f9cc6602d3f66b9c108549d475ec49e8ef4d45e # v4.0.0