Bug 1652086: [taskgraph] Don't compile schemas if we aren't going to check them; r=Callek

I'm planning on replacing voluptuous with something faster in Bug 1652123, but
in the meantime, compile the schema is slow, so there is no reason to do so if
we aren't going to check them anyway.

Differential Revision: https://phabricator.services.mozilla.com/D83540