shell.nix: remove deprecated stdenv.lib
shell.nix: remove deprecated stdenv.lib

`lib` is not related to the standard environment and thus `stdenv.lib`
has been deprecated and will not be included in upcoming
nixpkgs-releases. Instead, use it as `pkgs.lib` (top-level nixpkgs
attribute).

Signed-off-by: Leon Schuermann <be92910ae7bb6f58bb9b7c6f845bc30640119592@is.currently.online>

tock/libtock-c,tock/libtock-c,tock/libtock-c