Refactor execution to be able to use runSteps in exec mode directly
Created by: eseliger
To reduce the overhead of things that are unnecessary for exec mode, I've refactored the code to be able to reuse the RunSteps method directly. This will be helpful when designing a potential k8s adapter for executors that will need to be able to split the work of a single workspace across multiple jobs.
This yields a few nice side-effects:
- Net reduction of code
- It's more clear now which flags are actually required to run the execution so I could get rid of a few more things we had to configure but weren't actually used in the end
- Got rid of an unnecessary call to the workspace to get the changed files - turns out we already had a library function to transform a diff into that list.
Test plan
Test suite and manual testing.