Shadowing shell.nix binary

[soulomoon]

Issue description

I have some build tools binary to add in in my shell.nix.
It shadows build tools in shell.nix script if zsh's initExtra export the path contains the same binary.

[chisui]

Yes, this is a current limitation that I don't know how to fix without great effort. A workaround is to only add the binaries to the path if IN_NIX_SHELL is not set.

Also see: #19