ci: merge custom PATH directories

ci: merge custom PATH directories

We're downloading various executables required by our tests. Each of
these executables goes into its own directory, which is then appended to
the PATH variable. Consequently, whenever we add a new dependency and
thus a new directory, we would have to adapt to this change in several
places.

Refactor this to instead put all binaries into a single directory.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>