Normalize values used to represent true with boolean-esque env vars

Right now env vars that accept boolean-esque values (like DAPP_TEST_FFI, HEVM_RPC, and DAPP_BUILD_OPTIMIZE) each require slightly different values to represent true.

Sometimes 1 means true, sometimes yes means true, sometimes any non 0 value means true, and sometimes the variable being set at all means true!

Ideally all boolean-esque env vars would only accept one of these values to mean true, which would make using them a lot less confusing :p