rust: stop claiming the C index is compatible with the rust code This is no longer the case since the introduction of the pure Rust Index, and was probably not the case since the MixedIndex itself. So we fix the dedicated attribute value.

{
    "conduit_uri": "https://phab.mercurial-scm.org/api",
    "phabricator.uri": "https://phab.mercurial-scm.org/",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}