POC for using environment variables to override internal dependency branches

[mikealfare]

resolves #15

Problem

We need to be able to run integration tests, and possibly unit tests, against a branch other than main for internal dependencies (e.g. dbt-common, dbt-adapters).

Solution

Try creating dependency overrides based on environmental variables. The goal is to enable running this in GHA via inputs as well as running this locally by updating test.env.

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/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

[github-actions]

This PR has been marked as Stale because it has been open with no activity as of late. If you would like the PR to remain open, please comment on the PR or else it will be closed in 7 days.

[github-actions]

Although we are closing this PR as stale, it can still be reopened to continue development. Just add a comment to notify the maintainers.