ci: make failure to find perforce more user friendly

ci: make failure to find perforce more user friendly

In preparation for a future change that will make perforce installation
optional in macOS, make sure that the check for it is done without
triggering scary looking errors and add a user friendly message instead.

All other existing uses of 'type <cmd>' in our shell scripts that
check the availability of a command <cmd> send both standard output
and error stream to /dev/null to squelch "<cmd> not found" diagnostic
output, but this script left the standard error stream shown.

Redirect it just like everybody else to squelch this error message that
we fully expect to see.

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