remove "Using password from environment variable MINA_PRIVKEY_PASS" error from helper processes

[QuiteStochastic]

in the integration test framework, one commonly sees these errors coming from each of the nodes

        [2022-05-04 21:15:33.492438Z] {"timestamp":"2022-05-04 21:15:33.492438Z","level":"Error","source":{"module":"Verifier__Prod","location":"File \"src/lib/verifier/prod.ml\", line 306, characters 14-26"},"message":"Verifier stderr: $stderr","metadata":{"pid":17,"stderr":"Using password from environment variable MINA_PRIVKEY_PASS\nUsing password from environment variable MINA_PRIVKEY_PASS\nUsing password from environment variable MINA_PRIVKEY_PASS\n"}}
        [2022-05-04 21:15:33.681098Z] {"timestamp":"2022-05-04 21:15:33.681098Z","level":"Error","source":{"module":"Vrf_evaluator","location":"File \"src/lib/vrf_evaluator/vrf_evaluator.ml\", line 420, characters 12-24"},"message":"Vrf_evaluator stderr: $stderr","metadata":{"pid":17,"stderr":"Using password from environment variable MINA_PRIVKEY_PASS\nUsing password from environment variable MINA_PRIVKEY_PASS\nUsing password from environment variable MINA_PRIVKEY_PASS\n"}}

These errors are currently being ignored, however that is not an ideal situation. It seems that these processes, the prover, the verifier, the vrf evaluator, don't even need the privkey pass, and so they should be changed so that this error is removed.

[QuiteStochastic]

#10865 (review)