feat: optional path build.yaml option

[techouse]

Supersedes #20 originally authored by @marcel-ploch

---

Adds option for supporting the following build.yaml

targets:
  $default:
    builders:
      envied_generator|envied:
        options:
          path: .env.test
          override: true 

Only when BOTH override is true and the path is set will the generator resort to using the path provided in this build.yaml.

[techouse]

@marcel-ploch can you throw an eye on it, please?

CC/ @petercinibulk @ThexXTURBOXx

[marcel-ploch]

Hey @techouse ,

the PR looks fine to me and does the same as my original implementation.

Kind regards

[techouse]

I'll add some more explicit tests for this later in the evening and mark it ready for review again after that.

[marcel-ploch]

I can help writing tests if you like.
Just tell me what sort of testcases you have in mind ^^

[techouse]

I somehow managed to close this by mistake 🙈

Anyhow, this is ready for review

CC/ @marcel-ploch @petercinibulk @ThexXTURBOXx