Auto merge of #10799 - Urgau:check-cfg-fix-config-deserialization, r=ehuss

Auto merge of #10799 - Urgau:check-cfg-fix-config-deserialization, r=ehuss

Fix deserialization of check-cfg in config.toml

When improving the check-cfg implementation in https://github.com/rust-lang/cargo/pull/10566 I changed the internal representation of `check_cfg` from multiple `bool` options to one `Option<(bool, bool, bool, bool)>` but I didn't realize until https://github.com/rust-lang/rust/issues/82450#issuecomment-1169836928 that the internal representation is actually somewhat public as it's used in the `[unstable]` in `.cargo/config.toml`.

And because TOML cannot represent tuples there is no way to set it from the `[unstable]` section. This PR fix this oversight by using a custom deserializer method similar to what was already done for `build-std`.