ci/lib.sh: update a comment about installed P4 and Git-LFS versions

ci/lib.sh: update a comment about installed P4 and Git-LFS versions

A comment in 'ci/lib.sh' claims that the "OS X build installs the
latest available versions" of P4 and Git-LFS, but since f2f47150
("ci: don't update Homebrew", 2019-07-03) that's no longer the case,
as it will install the versions which were recorded in the image's
Homebrew database when the image was created.

Update this comment accordingly.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>