cosmic-comp: install default config files

There's a Makefile that installs the Rust binary and some default
config files.

pkgs/by-name/co/cosmic-comp/package.nix

@@ -77,6 +77,14 @@ rustPlatform.buildRustPackage rec {
     "-Wl,--pop-state"
   ];
 
+  makeFlags = [
+    "prefix=$(out)"
+    "CARGO_TARGET_DIR=target/${stdenv.hostPlatform.rust.cargoShortTarget}"
+  ];
+
+  # Use default stdenv installPhase, not the buildRustPackage one.
+  installPhase = "installPhase";
+
   # These libraries are only used by the X11 backend, which will not
   # be the common case, so just make them available, don't link them.
   postInstall = ''