Get mozilla overlay to work with cargo doc

I opened this issue a while ago and didn’t get any response so trying here.

Basically, I’m making a nix-shell for a rust project targeting an embedded arm target. While mostly the toolchain works and I can install packages with cargo, compile and test my program just fine, for some reason cargo-doc doesn’t work. Has anyone else experienced this issue or can someone point out what I am missing in my shell.nix to get it to work?