guix: use MULTIPROCESS (broken)

This shortcut doesn't work, because there's no curl in guix.

contrib/guix/libexec/build.sh

@@ -169,6 +169,7 @@ export TZ="UTC"
 
 # Build the depends tree, overriding variables that assume multilib gcc
 make -C depends --jobs="$JOBS" HOST="$HOST" \
+                                   MULTIPROCESS=1 \
                                    ${V:+V=1} \
                                    ${SOURCES_PATH+SOURCES_PATH="$SOURCES_PATH"} \
                                    ${BASE_CACHE+BASE_CACHE="$BASE_CACHE"} \