ci/install-dependencies: handle "sparse" job package installs

ci/install-dependencies: handle "sparse" job package installs

This just matches the style/location of the package installation for
other jobs. There should be no functional change.

I did flip the order of the options and command-name ("-y update"
instead of "update -y") for consistency with other lines in the same
file.

Note also that we have to reorder the dependency install with the
"checkout" action, so that we actually have the "ci" scripts available.

Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>