Fix `--rustc-cfg` not working
Fix `--rustc-cfg` not working

The name of the flag changed for some subcommands from `--cfg` to
`--rustc-cfg` but the parsing of the arguments was not updated to
account for the different names.

eminence/cargo,rust-lang/cargo,eminence/cargo,rust-lang/cargo,nipunn1313/cargo,eminence/cargo,rust-lang/cargo,rustyhorde/cargo,rust-lang/cargo,rustyhorde/cargo,nipunn1313/cargo,eminence/cargo,nipunn1313/cargo,rustyhorde/cargo,nipunn1313/cargo,rustyhorde/cargo