fix: comment-in and modify build_jobs only once in spack config.yaml

* newer spack breaks when it is specified more than once
  like four times currently because also all explanation in the
  comment is modified

Change-Id: I3f6c287e7ac77d5526ec7f3256f1cd75e5215bf8

.ci/bootstrap_spack.sh

@@ -26,7 +26,7 @@ ccache -s
 sed -i '/ccache:/c\  ccache: true' "${MY_SPACK_FOLDER}/etc/spack/defaults/config.yaml"
 
 # build with all available processes
-sed -i "/build_jobs:/c\  build_jobs: $(nproc)" "${MY_SPACK_FOLDER}/etc/spack/defaults/config.yaml"
+sed -i "/# build_jobs:/c\  build_jobs: $(nproc)" "${MY_SPACK_FOLDER}/etc/spack/defaults/config.yaml"
 
 # add system compiler
 ${MY_SPACK_BIN} compiler add --scope site /usr/bin