nix: use user package set on overlay and switch to ninja

flake.nix

@@ -27,7 +27,11 @@
         };
 
         overlay = (final: prev: rec { 
-          polymc = packages.polymc;
+          polymc = prev.libsForQt5.callPackage ./packages/nix/polymc {
+            inherit self;
+            submoduleQuazip = quazip;
+            submoduleNbt = libnbtplusplus;
+          };
         });
 
         apps = {

packages/nix/polymc/default.nix

@@ -5,6 +5,7 @@
 , substituteAll
 , fetchpatch
 , cmake
+, ninja
 , jdk8
 , jdk
 , zlib
@@ -38,7 +39,7 @@ mkDerivation rec {
 
   src = lib.cleanSource self;
 
-  nativeBuildInputs = [ cmake file makeWrapper ];
+  nativeBuildInputs = [ cmake ninja file makeWrapper ];
   buildInputs = [ qtbase jdk8 zlib ];
 
   postUnpack = ''
@@ -50,6 +51,7 @@ mkDerivation rec {
   '';
 
   cmakeFlags = [
+    "-GNinja"
     "-DLauncher_LAYOUT=lin-system"
   ];