github: simplify computation of the job's distro

github: simplify computation of the job's distro

We explicitly list the distro of Linux-based jobs, but it is equivalent
to the name of the image in almost all cases, except that colons are
replaced with dashes. Drop the redundant information and massage it in
our CI scripts, which is equivalent to how we do it in GitLab CI.

There are a couple of exceptions:

  - The "linux32" job, whose distro name is different than the image
    name. This is handled by adapting all sites to use the new name.

  - The "alpine" and "fedora" jobs, neither of which specify a tag for
    their image. This is handled by adding the "latest" tag.

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