automation: use m6i instances
This instance type is much, much faster than t3 and can perform
tasks much quicker.
Differential Revision: https://phab.mercurial-scm.org/D12131
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}