t/lib-pager: use sane_unset() to avoid breaking &&-chain

t/lib-pager: use sane_unset() to avoid breaking &&-chain

This test intentionally breaks the &&-chain following `unset` since it
doesn't know if `unset` will succeed or fail and doesn't want a local
`unset` failure to abort the test overall. We can do better by using
sane_unset() which can be linked into the &&-chain as usual.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>