ci: deduplicate calls to `apt-get update`

ci: deduplicate calls to `apt-get update`

When installing dependencies we first check for the distribution that is
in use and then we check for the specific job. In the first step we
already install all dependencies required to build and test Git, whereas
the second step installs a couple of additional dependencies that are
only required to perform job-specific tasks.

In both steps we use `apt-get update` to update our repository sources.
This is unnecessary though: all platforms that use Aptitude would have
already executed this command in the distro-specific step anyway.

Drop the redundant calls.

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