.arcconfig

examples: specify rustfmt nightly using a $() construct

This is ugly, but it's how we have to configure rustfmt for now as we
require nightly rustfmt.

Differential Revision: https://phab.mercurial-scm.org/D7812

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