Use --check-cfg to validate cfg attributes during automated builds

[rtzoeller]

Consider enabling some form of the --check-cfg flag during automated builds, to catch typos.

rust-lang/cargo#10554

[SteveLauC]

#2293 is a duplicate of this issue, but I am going to close this one as the new one has more information and is up-to-date with the current Nix status:)