ci: use https, not http to download binaries from perforce.com

ci: use https, not http to download binaries from perforce.com

Since 522354d70f4 (Add Travis CI support, 2015-11-27) the CI has used
http://filehost.perforce.com/perforce/ to download binaries from
filehost.perforce.com, they were then moved to this script in
657343a602e (travis-ci: move Travis CI code into dedicated scripts,
2017-09-10).

Let's use https instead for good measure. I don't think we need to
worry about the DNS or network between the GitHub CI and perforce.com
being MitM'd, but using https gives us extra validation of the payload
at least, and is one less thing to worry about when checking where
else we rely on non-TLS'd http connections.

Also, use the same download site at perforce.com for Linux and macOS
tarballs for consistency.

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