Don't consult or change the global and system git configuration

What does this MR do?

Writes a hard-coded gitconfig file per build and sets it in $GIT_CONFIG.

Why was this MR needed?

Previously, we were running `git config --global fetch.recurseSubmodules false" to disable recursive submodule fetches. This has a few problems:

• It modifies global state for the user the runner is under, which is unexpected in some environments
• Concurrent builds sometimes fail as git config doesn't cleanly handle concurrent writes to the same file.

This change switches to a separate gitconfig file per-build, with hardcoded values. This solves both issues, at the cost of removing support for setting custom options for the runner in /etc/gitconfig or ~/.gitconfig.

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

• Do we need to support /etc/gitconfig or ~/.gitconfig (or the Windows equivalents)? Do we know if users are relying on this secret feature at the moment?

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• 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?

Closes #1469 (closed)