chore(tvix/shell): add cargo-expand to debug proc-macro

I lost a lot of hope and had to read the source code of `quote!`, `cargo expand` was invaluable
in this adventure. We should keep it IMHO.

Change-Id: Icfb4c80d413602f2bdc6deab0d595183825d88ad
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10635
Autosubmit: raitobezarius <tvl@lahfa.xyz>
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/shell.nix

@@ -18,6 +18,7 @@ pkgs.mkShell {
     pkgs.buf-language-server
     pkgs.cargo
     pkgs.cargo-machete
+    pkgs.cargo-expand
     pkgs.clippy
     pkgs.evans
     pkgs.fuse