buildLuarocksPackage: rework fixup phase

I was working on lua tests, enabling nlua and I started overriding
postFixup, which removed the default phase for build-luarocks-package so
instead let's make it a proper phase.

pkgs/development/interpreters/lua-5/build-luarocks-package.nix

@@ -94,6 +94,7 @@ let
   ];
 
   inherit doCheck extraConfig rockspecFilename knownRockspec externalDeps nativeCheckInputs;
+  inherit dontWrapLuaPrograms;
 
   buildInputs = let
     # example externalDeps': [ { name = "CRYPTO"; dep = pkgs.openssl; } ]
@@ -177,9 +178,11 @@ let
     runHook postBuild
   '';
 
-  postFixup = lib.optionalString (!dontWrapLuaPrograms) ''
-    wrapLuaPrograms
-  '' + attrs.postFixup or "";
+  fixupPhase = ''
+    runHook preFixup
+    ${lib.optionalString (!self.dontWrapLuaPrograms) "wrapLuaPrograms"}
+    runHook postFixup
+    '';
 
   installPhase = ''
     runHook preInstall
@@ -196,6 +199,7 @@ let
     # maybe we could reestablish dependency checking via passing --rock-trees
 
     nix_debug "ROCKSPEC $rockspecFilename"
+    # deps-mode=all tells luarocks to use every configured rocks_trees
     luarocks $LUAROCKS_EXTRA_ARGS make --deps-mode=all --tree=$out ''${rockspecFilename}
 
     runHook postInstall