Unset GIT_SSL_CAINFO before syncing submodules

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What does this MR do?

Unset GIT_SSL_CAINFO before syncing submodules.

Why was this MR needed?

There might be submodules from HTTPS sites other than the set-up GitLab, so GIT_SSL_CAINFO has to be unset before syncing submodules in case git complains about untrusted certificates.

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

Since I have no access to a Windows system, I would love to have WIndows users confirm that variable unsetting methods of cmd and powershell do do the job.

Does this MR meet the acceptance criteria?

  • Documentation created/updated
  • 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?