ENV variables for motoko builds

[paulyoung]

You can probably do that by using the custom canister type in dfx.json. Something like;

{
  "canisters": {
    "foo": {
      "type": "custom",
      "build": "moc -c …",
      "candid": "src/foo/src/foo.did",
      "wasm": "foo.wasm"
    }
  }
}