outputs: remove selfPath, but use that method in selfExpr default

by stringifying ./. then appending path, nix adds selfPath (nix-store path) to gcroot of selfPath derivation

pkgs/overlays/selfExpr.nix

@@ -25,7 +25,7 @@ self: super: {
     in ''
       { ... } @ args:
 
-      import ${builtins.toString nixpkgsPath} {
+      import ${nixpkgsPath} {
         ${self.lib.optionalString useConfig configText}
       } // builtins.removeAttrs args (builtins.fromJSON '''
         ${removedAttrNamesText}