refactor(readTree): Move copy of 'fix' into readTree

This is often used when bootstrapping a repository with readTree,
before lib is available. Having this definition in readTree is more
convenient than copy&pasting it around to callsites.

Change-Id: I6d5d27ed142bea704843fe289ad2674be8c4d360

nix/readTree/default.nix

@@ -169,4 +169,11 @@ in {
         '';
       };
     };
+
+  # This definition of fix is identical to <nixpkgs>.lib.fix, but is
+  # provided here for cases where readTree is used before nixpkgs can
+  # be imported.
+  #
+  # It is often required to create the args attribute set.
+  fix = f: let x = f x; in x;
 }