contrib: adjust heptapod CI flavor sizes
Our CI currently runs on a number of different runners, some of which are run
by Clever Cloud.
By setting this environement variable, we're asking for a smaller size runner
for our smaller jobs, which puts less load on the Clever Cloud infrastructure,
while not affecting our overall pipeline times.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}