Add tests and documentation of support for `env` key for `schedules`

Related to:

• https://github.com/meltano/meltano/issues/6386

schedules:
- name: gitlab
  interval: '@daily'
  job: gitlab
  env:
    DBT_MODELS: +gitlab+

Precedence discussion in https://github.com/meltano/meltano/issues/6386#issuecomment-1179138398

[cjohnhanson]

@tayloramurphy I think it’s reasonable to include fixing the behavior in the scope of this issue. With adding testing for other levels of env support we were starting from scratch, whereas here it’s more a matter of adding test cases than writing an entire new test suite from the ground up.

[tayloramurphy]

@kgpayne @cjohnhanson you’ve both had a look at the environment code:

• Can you confirm that env is a valid top-level key for schedules?
• Can you confirm that its inheritance is in line with what we’re discussing in https://github.com/meltano/meltano/issues/6386 ?
• Can you confirm if this is indeed tested or not currently?

Note I’m adding this to Iteration 8 because not having this documented (and possibly not tested) is a big risk for users.

Edit: Seems like it is documented in https://docs.meltano.com/guide/integration#pipeline-specific-configuration but that’s a weird place for the only mention of it to be!