Bug 1888321 - Rename .cargo/config.in to .cargo/config.toml.in. r=firefox-build-system-reviewers,webdriver-reviewers,ahochheiden

Cargo 1.79 is emitting a warning when using .cargo/config.
.cargo/config.toml has been supported since cargo 1.39.

Differential Revision: https://phabricator.services.mozilla.com/D205974