redprl: remove explicit builder

patchPhase wasn't being called for me so I've switched out the explicit
builder for the generic form. I can now build RedPRL on locally on
NixOS.

pkgs/applications/science/logic/redprl/default.nix

@@ -11,12 +11,11 @@ stdenv.mkDerivation {
   patchPhase = ''
     patchShebangs ./script/
   '';
-  builder = builtins.toFile "builder.sh" ''
-    source $stdenv/setup
-    mkdir -p $out/bin
-    cp -r $src/* .
-    chmod -R +w src
+  buildPhase = ''
     ./script/mlton.sh
+  '';
+  installPhase = ''
+    mkdir -p $out/bin
     mv ./bin/redprl $out/bin
   '';
   meta = {