ci: Reduce the number of integration tests we run concurrently

Since there's a bunch of CPU hungry systemd-journal-remote processes
running on the host to received the forwarded logs, by running as many
test as the VM has cores we overload the available resources. Let's leave
use the number of cores - 1 to reduce resource contention.

.github/workflows/mkosi.yml

@@ -180,7 +180,7 @@ jobs:
       run: meson compile -C build mkosi
 
     - name: Run integration tests
-      run: meson test -C build --no-rebuild --suite integration-tests --print-errorlogs --no-stdsplit
+      run: meson test -C build --no-rebuild --suite integration-tests --print-errorlogs --no-stdsplit --num-processes "$(($(nproc) - 1))"
 
     - name: Archive failed test journals
       uses: actions/upload-artifact@v4