codespaces: account for github/codespaces `--system` prefix

[pmrowla]

Codespaces sets user.name/email for the github user in a non-standard --system level config. If we fail to get a valid signature (user.name/user.email) when generating git commits, we need to check for codespaces env vars and then manually load the signature from /usr/local/etc/gitconfig if we are in a codespaces env

(libgit2/pygit2 and dulwich will only check /etc/gitconfig which is the standard --system config location)

related: community/community#38070

to be clear this is a workaround for codespaces specific containers that should be removed if/when the codespaces behavior changes

[pmrowla]

this workaround is available in 0.1.3, leaving this issue open so we don't forget to remove it whenever the upstream github issue is resolved