ci: disable Homebrew's auto cleanup

ci: disable Homebrew's auto cleanup

Lately Homebrew learned to automagically clean up information about
outdated packages during other 'brew' commands, which might be useful
for the avarage user, but is a waste of time in CI build jobs, because
the next build jobs will start from the exact same image containing
the same outdated packages anyway.

Export HOMEBREW_NO_INSTALL_CLEANUP=1 to disable this auto cleanup feature,
shaving off about 20-30s from the time needed to install dependencies
in our macOS build jobs on Travis CI.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>