ci: accelerate the checkout

ci: accelerate the checkout

By upgrading from v1 to v2 of `actions/checkout`, we avoid fetching all
the tags and the complete history: v2 only fetches one revision by
default. This should make things a lot faster.

Note that `actions/checkout@v2` seems to be incompatible with running in
containers: https://github.com/actions/checkout/issues/151. Therefore,
we stick with v1 there.

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