Use parallel make in Linux build workflow for improved build speed

Co-authored-by: rolfbjarne <249268+rolfbjarne@users.noreply.github.com>

Author: Copilot

.github/workflows/linux-build.yml

@@ -26,4 +26,4 @@ jobs:
       - name: Build on Linux
         run: |
           set -ex
-          make
+          make -j$(nproc)