build: use generic builder in urbit derivation

'installPhase' captures all the important stuff that builder.sh
previously did, and use of the generic builder allows stages of the
build to be more easily overridden.

nix/pkgs/urbit/builder.sh

@@ -1,15 +0,0 @@
-source $stdenv/setup
-
-cp -r $src ./src
-chmod -R u+w ./src
-cd src
-
-bash ./configure
-
-make clean
-make all -j8
-make test
-
-mkdir -p $out/bin
-cp ./build/urbit $out/bin/$exename
-cp ./build/urbit-worker $out/bin/$exename-worker

nix/pkgs/urbit/default.nix

@@ -27,9 +27,17 @@ let
     inherit name meta;
     exename = name;
     src     = ../../../pkg/urbit;
-    builder = ./builder.sh;
     nativeBuildInputs = deps ++ vendor;
 
+    installPhase = ''
+      make all -j8
+      make test
+
+      mkdir -p $out/bin
+      cp ./build/urbit $out/bin/$exename
+      cp ./build/urbit-worker $out/bin/$exename-worker
+    '';
+
     # See https://github.com/NixOS/nixpkgs/issues/18995
     hardeningDisable = if debug then [ "all" ] else [];