run-tests: prevent race-condition when picking a channel
Before this, multiple jobs could search the list at the same time and pick the
same free channel.
We now project this search/assignment with a simple lock.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}