CI: Parallelize make commands

Can the make commands in ci/build_project.yml use -j$(nproc) to speed up builds?

ci/build_project.yml:148:    - $WRAPPER make 2>&1 | tee build.log
ci/build_project.yml:164:    - DISTCHECK_CONFIGURE_FLAGS="$CONFIGURE_FLAGS" make distcheck

I'm not sure if serial execution is required for certain projects, but parallelization should help speed up Thunar's checks: https://gitlab.xfce.org/xfce/thunar/-/pipelines/30464/builds