rust: use `.cargo/config.toml` instead of `.cargo/config`
This has been deprecated for a while now and we don't support Rust versions
that only understand the old path.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}