Support multiple GitHub SSH deploy keys

[eronnen]

complementary PR to https://github.com/game-ci/unity-builder/pull/568

Changes

• add sshPublicKeysDirectoryPath input, which is a custom .ssh that should contain the public keys and config file.
• pass a GIT_CONFIG_EXTENSIONS environment variable to the container, which is a list of key=values configs that can be retrieved from git config --list that the git inside the container will use.

Checklist

• [x] Read the contribution guide and accept the code of conduct
• [ ] Readme (updated or not needed)
• [ ] Tests (added, updated or not needed)

[github-actions[bot]]