A source build against a private repository with no FELIS_GITHUB_TOKEN,
or a wrong one, made git ask for a username on /dev/tty, and a piped
install sat there waiting.
git_auth now runs git with GIT_TERMINAL_PROMPT=0 on both arms, so git
fails at once with "terminal prompts disabled". Both fetch_source
failures name FELIS_GITHUB_TOKEN in their message: the fresh clone,
and the fetch into an existing checkout, which had no message of its
own before.
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