rrsync: correct meta.mainProgram

rrsync.meta inherits from rsync.meta, so #248417 had the effect of
setting rrsync.meta.mainProgram to "rsync", which doesn’t exist:

    /nix/store/…-rrsync-3.3.0/bin/rsync: No such file or directory

Re #298509

pkgs/applications/networking/sync/rsync/rrsync.nix

@@ -27,5 +27,6 @@ stdenv.mkDerivation {
 
   meta = rsync.meta // {
     description = "Helper to run rsync-only environments from ssh-logins";
+    mainProgram = "rrsync";
   };
 }