etc: Add explicit ‘--substitute-urls’ in guix-daemon service files.

Having substitute URLs explicitly listed in the service startup file
makes it clearer what should be modified to permanently change the list
of substitute URLs.

* config-daemon.ac: Rename ‘guix_substitute_urls’ to
‘GUIX_SUBSTITUTE_URLS’ and substitute it.
* nix/local.mk (etc/guix-%.service, etc/init.d/guix-daemon)
(etc/guix-%.conf): Substitute it.
* etc/guix-daemon.conf.in, etc/guix-daemon.service.in,
etc/init.d/guix-daemon.in: Add an explicit ‘--substitute-urls’ option.

Change-Id: Ie491b7fab5c42e54dca582801c03805a85de2bf9

etc/init.d/guix-daemon.in

@@ -36,7 +36,9 @@ start)
       -E LC_ALL=en_US.utf8 \
       -p "/var/run/guix-daemon.pid" \
       @localstatedir@/guix/profiles/per-user/root/current-guix/bin/guix-daemon \
-      --build-users-group=guixbuild --discover=no
+      --build-users-group=guixbuild --discover=no \
+      --substitute-urls='@GUIX_SUBSTITUTE_URLS@'
+
   fi
   ;;
 stop)