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

Start by locating how DAPP_TEST_FFI, HEVM_RPC, and DAPP_BUILD_OPTIMIZE are parsed and compare the values currently treated as true. Then inventory the other boolean-esque environment variables and determine whether they follow the same normalization rule. Done means the relevant variables consistently accept the agreed true representation and their behavior is covered by the existing tests.

Written by the indexing model from the issue text.