Add BOM to make powershell happy with utf-8 characters.

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What does this MR do?

Add BOM to generated powershell script, making powershell correctly recognize the encoding as utf-8.

Why was this MR needed?

Non-ASCII character in GITLAB_USER_NAME may cause build to failed without this patch.

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

The "\uFEFF" will be correctly translate to byte sequence 0xEF,0xBB,0xBF when writing to the script.

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?

Not exists.