make it easy to configure cargo profiles from nix

This way you can easily build in debug mode with Nix.

nix/pkgs/default/default.nix

@@ -10,6 +10,7 @@
 # Options (keep sorted)
 , default-features ? true
 , features ? []
+, profile ? "release"
 }:
 
 let
@@ -73,6 +74,8 @@ craneLib.buildPackage rec {
     pkgsBuildHost.rustPlatform.bindgenHook
   ];
 
+  CARGO_PROFILE = profile;
+
   inherit env;
 
   passthru = {