run-tests: detect HGWITHRUSTEXT value
Without this, some manual check in tests/test-dirstate.t could get confused by
the lack of `rust` in module policy and break the test.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}