nixguix: Make the unsupported artifact extensions configurable
So the maintenance is easier (as in, sometimes, it could be an update of just one extension to add/remove and not necessarily code to change, which would then imply some extra package and deploy work).
Note that if this lands, this needs an update on the current loader-nixguix's configuration puppet wise (swh/infra/puppet/puppet-swh-site!216 (closed))
Test Plan
tox
Migrated from D3745 (view on Phabricator)