untrunc-anthwlock: refactor build
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).

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs