stdenvAdapters.keepDebugInfo: add Rust support

These are the flags we use in the separateDebugInfo hook, plus
disabling optimisations in line with what we do for C.

This fixes e.g. enableDebugging amberol.

pkgs/stdenv/adapters.nix

@@ -295,6 +295,7 @@ rec {
         dontStrip = true;
         env = (args.env or { }) // {
           NIX_CFLAGS_COMPILE = toString (args.env.NIX_CFLAGS_COMPILE or "") + " -ggdb -Og";
+          NIX_RUSTFLAGS = toString (args.env.NIX_RUSTFLAGS or "") + " -g -C opt-level=0 -C strip=none";
         };
       });
     });