nix: Base overridden packages on correct package versions

Previously we would use whatever version was in nixpkgs yet this may not
be the version which the patch is supposed to be applied against.

scripts/generate-nix-overrides.hs

@@ -53,9 +53,11 @@ generateOverride prefix patchDir (patchExtless, patchTypes) = do
   pid <- maybe (fail ("invalid patch file name: " ++ show patchExtless)) return pid'
   let pname = display (packageName pid)
       version = versionNumbers (packageVersion pid)
+      --baseDrv = "super."++pname
+      baseDrv = "(super.callHackage \""++pname++"\" \""++intercalate "." (map show version)++"\" {})"
   return . (packageName pid,) . (version,) $ 
     "haskell.lib.doJailbreak (dontRevise "
-    ++ foldl' (override prefix patchDir patchExtless) ("super."++pname) patchTypes ++ ")"
+    ++ foldl' (override prefix patchDir patchExtless) baseDrv patchTypes ++ ")"
 
 patchFunction :: PatchType -> String
 patchFunction = \case