t/gpg: simplify test for unknown key

t/gpg: simplify test for unknown key

To test for a key that is completely unknown to the keyring we need one
to sign the commit with. This was done by generating a new key and not
add it into the keyring. To avoid the key generation overhead and
problems where GPG did hang in CI during it, switch GNUPGHOME to the
empty $GNUPGHOME_NOT_USED instead, therefore making all used keys unknown
for this single `verify-commit` call.

Reported-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Fabian Stelzer <redacted>
Reviewed-by: Taylor Blau <redacted>
Signed-off-by: Junio C Hamano <redacted>