Refactor get sources script building

What does this MR do?

Refactors a little how the get sources script is being built in shells/abstract.go. It should not change the build script, but only update the way how it is done (to make it easier to handle #4304 (closed) on top of it).

This was extracted from !1297 (closed) to make it easier to review.

Why was this MR needed?

Second step to achieve #4304 (closed).

Are there points in the code the reviewer needs to double check?

⚠ Prepared on top of !1481 (merged). Requires it to be merged first ⚠

After merging !1481 (merged) we should change the target branch here to master and then un-WIP and merge this one.

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Added tests for this feature/bug
• In case of conflicts with master - branch was rebased

What are the relevant issue numbers?

References #4304 (closed), #4123 (closed)