Fix nix-shell: move pkg-config to nativeBuildInputs

[Familex]

Cargo was throwing this error during build:

...
error: could not find system library 'openssl' required by the 'openssl-sys' crate

--- stderr
Package openssl was not found in the pkg-config search path.
Perhaps you should add the directory containing `openssl.pc'
to the PKG_CONFIG_PATH environment variable
Package 'openssl' not found

...

It looks like you're compiling on Linux and also targeting Linux. Currently this
requires the `pkg-config` utility to find OpenSSL but unfortunately `pkg-config`
could not be found. If you have OpenSSL installed you can likely fix this by
installing `pkg-config`.
...

I moved pkg-config from buildInputs to nativeBuildInputs, adding it to PATH.

That fixed the issue for me, but I don't understand why shell.nix was written that way.
@Builditluc, was pkg-config installed on your host system?

[Builditluc]

Hey @Familex, please excuse my late reply. Thank you for updating the shell.nix file.

why shell.nix was written that way

Well, tbh. the file worked for me at least until last year (could also be that I installed pkg-config on my host system).