'Support for GitHub deploy keys' breaks local CI for repositories which are submodules of other repositories which use ssh-agent with local CI

[AlistairMcCutcheonIAS]

Versions:

• actions/checkout@v4
• webfactory/ssh-agent@v0.7.0

How to reproduce:

1. Create two repositories, repo_1 and repo_2.
2. Make repo_1 use repo_2 as a submodule as per https://github.com/webfactory/ssh-agent?tab=readme-ov-file#support-for-github-deploy-keys
3. Run CI on repo_1 on a local runner. This will add the url..insteadof to the global .gitconfig of that runner.
4. Run CI on repo_2 on the same local runner. When trying to checkout the repository, the checkout will be redirected to use the deploy key in ~/.ssh/ (added by repo_1's CI) rather than the PAT. This will cause the checkout action to fail, with the following error:

Fetching the repository
  /usr/bin/git -c protocol.version=2 fetch --no-tags --prune --progress --no-recurse-submodules --depth=1 origin +9849...41:refs/remotes/pull/7/merge
  Load key "/home/docker/.ssh/key-3e0...9f[40](https://github.com/org/repo/actions/runs/87...80/job/2...5#step:2:45)b75...5d9": invalid format
  git@github.com: Permission denied (publickey).
  Error: fatal: Could not read from remote repository.

Workaround:

• Deleting the global .gitconfig before each run stops the issue

[dcasnowdon]

Something similar is affecting different workflows running on the same local runner.