feat: Implement env parsing

[pawelpasterz]

Fixes #1882

Test Plan

How do we know the code works?

1. run ./gradlew flankFullRun
2. export env variable with test targets, i.e.

   export TEST_TARGETS="class your.test.Clazz, notAnnotation your.any.Annotation"
   

3. update your config with env:

   test-targets:
     - $TEST_TARGETS
   

4. start flank test run and see test targets being applied

Checklist

• Documented
• Unit tested

[pawelpasterz]

@flank-it

[github-actions]

Integration tests succeed for all OSs ✅
Windows Build scan:
MacOS Build scan: https://gradle.com/s/d6kzrrg6tvlk4
Linux Build scan: https://gradle.com/s/t74vtyofeefwy
Workflow run https://github.com/Flank/flank/actions/runs/1122873168

[github-actions]

Timestamp: 2021-08-13 11:27:33
Buildscan url for ubuntu-workflow run 1127453686
https://gradle.com/s/yjmjzew5yuyyu