fix(guix-improve): pass JOBS to guix pull and system image

steps-guix/improve/guix-build-iso.sh

@@ -22,6 +22,7 @@ mkdir -p "${out_dir}"
 iso_store_path="$(guix system image \
     --system=x86_64-linux \
     -t iso9660 \
+    -c "${JOBS}" \
     -e '(@@ (gnu system install) installation-os)' \
     --no-substitutes \
     --disable-authentication)"

steps-guix/improve/guix-daemon-and-pull.sh

@@ -295,4 +295,4 @@ if ! guile -c '(use-modules (gnutls)) (if (module-variable (resolve-module (quot
 fi
 
 start_distfiles_http_server
-guix pull --bootstrap --no-substitutes --channels="${channels_file}" --disable-authentication
+guix pull --bootstrap -c "${JOBS}" --no-substitutes --channels="${channels_file}" --disable-authentication