pythonPackages.pip: force installing

pkgs/top-level/python-packages.nix

@@ -15067,6 +15067,10 @@ in modules // {
       sha256 = "46f4bd0d8dfd51125a554568d646fe4200a3c2c6c36b9f2d06d2212148439521";
     };
 
+    # pip detects that we already have bootstrapped_pip "installed", so we need
+    # to force it a little.
+    installFlags = [ "--ignore-installed" ];
+
     buildInputs = with self; [ mock scripttest virtualenv pytest ];
   };