Currently, tests fail when the host system has `commit.gpgsign` or
`tag.gpgsign` enabled at the system level (in my case, a custom path
located at `/etc/git/config`), since the signing key is unavailable in
the test sandbox.
The tests set `HOME=$TEST_HOME` to isolate themselves, which bypasses
the user-level git config (`~/.gitconfig`). However, if a user sets the
system-level config via `GIT_CONFIG_GLOBAL` or `GIT_CONFIG_SYSTEM`, it
still applies, causing commits to fail when signing is enabled there.
In this PR, I explicitly set `GIT_CONFIG_GLOBAL` and `GIT_CONFIG_SYSTEM`
to `/dev/null` so that the user's system config is never read from or
written to. I've also replaced `git config --global protocol.file.allow
always` with `GIT_CONFIG_*` environment variables to avoid writing to
`/dev/null`.
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