users/s6-rc: split out compiled var

modules/users/s6-rc.nix

@@ -226,10 +226,11 @@ in
           s6SvcsFromConfigServices config.services
         )
       );
+      compiled = compileServices sources;
     in {
       # N.B.: `compiled` needs to be writable (for locks -- maybe i can use symlinks to dodge this someday):
       # i populate it to ~/.config as a well-known place, and then copy it to /run before actually using it live.
-      fs.".config/s6/compiled".symlink.target = compileServices sources;
+      fs.".config/s6/compiled".symlink.target = compiled;
       # exposed only for convenience
       fs.".config/s6/sources".symlink.target = sources;