Introduce DBT_SELECTOR as environment variable for --selector

[fornwall]

Resolves #12193

Problem

It's not possible to specify a named selector for dbt runs using an environment variable.

Solution

Introduce support for DBT_SELECTOR.

Checklist

• I have read the contributing guide and understand what's expected of me.
• I have run this code in development, and it appears to resolve the stated issue.
• This PR includes tests, or tests are not required or relevant for this PR.
• This PR has no interface changes (e.g., macros, CLI, logs, JSON artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX.
• This PR includes type annotations for new and modified functions.