Run only `application/sql` on loadtests

So perhaps we should modify the loadtests to not run the SQL at all? One idea could be to have them return application/sql (#3580) so they still go through the whole Haskell pipeline.

Yeah, I like that :rocket:. That fits both my mental model of what the loadtests should do, plus it should certainly help to reduce noise in CI.

Originally posted by in https://github.com/PostgREST/postgrest/pull/5020#discussion_r3477432438