mu: use absolute path to mu in activation block

modules/programs/mu.nix

@@ -52,7 +52,7 @@ in {
       # In theory, mu is the only thing that creates that directory, and it is
       # only created during the initial index.
       if [[ ! -d "${dbLocation}" ]]; then
-        $DRY_RUN_CMD mu init ${maildirOption} ${myAddresses} $VERBOSE_ARG;
+        $DRY_RUN_CMD ${pkgs.mu}/bin/mu init ${maildirOption} ${myAddresses} $VERBOSE_ARG;
       fi
     '';
   };