Remove shallow.lock before fetching

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What does this MR do?

Removes .git/shallow.lock prior to fetching.

Why was this MR needed?

Cancelled builds mid-shallow clone/fetch leave behind this file and prevent rebuilds with the 'fetch' strategy.

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

Does this MR meet the acceptance criteria?

  • Tests
    • Added for this feature/bug
    • All builds are passing
  • Branch has no merge conflicts with master (if you do - rebase it please)

What are the relevant issue numbers?

#1481 (closed)

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