ci: avoid brew for installing perforce

ci: avoid brew for installing perforce

Perfoce's cask in brew is meant[1] to be used only by humans, so replace
its use from the CI with a scripted binary download which is less likely
to fail, as it is done in Linux.

Kept the logic together so it will be less likely to break when moved
around as on the fly code changes in this area are settled, at which
point it will also feasable to ammend it to avoid some of the hardcoded
values by using similar variables to the ones Linux does.

In that same line, a POSIX sh syntax is used instead of the similar one
used in Linux in preparation for an unrelated future change that might
change the shell currently configured for it.

This change reintroduces the risk that the installed binaries might not
work because of being quarantined that was fixed with 5ed9fc3fc86 (ci:
prevent `perforce` from being quarantined, 2020-02-27) but fixing that
now was also punted for simplicity and since the affected cloud provider
is scheduled to be retired with an on the fly change, but should be
addressed if that other change is not integrated further.

The discussion on the need to keep 2 radically different versions of
the binaries to be tested with Linux vs macOS or how to upgrade to
newer versions now that brew won't do that automatically for us has
been punted for now as well.  On that line the now obsolete comment
about it in lib.sh was originally being updated by this change but
created conflicts as it is moved around by other on the fly changes,
so will be addressed independently as well.

[1] https://github.com/Homebrew/homebrew-cask/pull/122347#discussion_r856026584

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>