[CT-348] Unit tests broken for release branches

[leahwicz]

For our unit tests, we wanted to make sure we were testing new changes against the newest version of dbt-core so that we could adapt to any changes made there (they are essentially our foundation). So for pushes, PR's, and just manual triggers, we clone the latest code from dbt-core and dbt-postgres on the main branch and use that to test against. The code for this can be found here.

This works well for dbt-redshift when we are testing out the main branch or changes going in to the main branch.

This doesn't work well when we are trying to test a release branch or changes going into a release branch. We would essentially be testing dbt-core code in main against dbt-redshift code that is for 1.0.latest. We should be testing against dbt-core's 1.0.latest branch instead.

We now have breaking changes on the dbt-core main branch that we incorporated into the dbt-redshift main branch but the dbt-redshift 1.0.latest branch is broken because it is referring to the newer dbt-core core.