untrunc-anthwlock: refactor build

Use buildPhase to make clear that we need two invocations of 'make' with
different targets.

Also set IS_RELEASE to 1 to ensure an optimized build is created instead
of a debug build (see Makefile).

pkgs/tools/video/untrunc-anthwlock/default.nix

···
     sha256 = "14i2lq68q990hnm2kkfamlsi67bcml85zl8yjsyxc5h8ncc2f3dp";
   };
 
-
   buildInputs = [ ffmpeg libui ];
 
-  postBuild = ''
-    make untrunc-gui
+  buildPhase = ''
+    runHook preBuild
+    make IS_RELEASE=1 untrunc
+    make IS_RELEASE=1 untrunc-gui
+    runHook postBuild
   '';
 
   installPhase = ''