ci: install test dependencies for linux-musl

ci: install test dependencies for linux-musl

The linux-musl CI job executes tests on Alpine Linux, which is based on
musl libc instead of glibc. We're missing some test dependencies though,
which causes us to skip a subset of tests.

Install these test dependencies to increase our test coverage on this
platform. There are still some missing test dependecies, but these do
not have a corresponding package in the Alpine repositories:

    - p4 and p4d, both parts of the Perforce version control system.

    - cvsps, which generates patch sets for CVS.

    - Subversion and the SVN::Core Perl library, the latter of which is
      not available in the Alpine repositories. While the tool itself is
      available, all Subversion-related tests are skipped without the
      SVN::Core Perl library anyway.

The Apache2-based tests require a bit more care though. For one, the
module path is different on Alpine Linux, which requires us to add it to
the list of known module paths to detect it. But second, the WebDAV
module on Alpine Linux is broken because it does not bundle the default
database backend [1]. We thus need to skip the WebDAV-based tests on
Alpine Linux for now.

[1]: https://gitlab.alpinelinux.org/alpine/aports/-/issues/13112

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