Revert "rust: Write config.toml not config"

[winterqt]

Reverts #321095, as it breaks Cargo FOD hashes.

[drupol]

Thank you for doing it @winterqt.

[chuangzhu]

Reverting is just a temporary mediation for the package breakages. You know, the concern #321095 raised

the latter (.cargo/config) is deprecated (since 1.38?) and is now warning on stderr.

is actually legit: if it throws an warning now, it could raise an error and refuse to build in the next rust release

[winterqt]

@chuangzhu See #321095 (comment) and #321095 (comment).