CI: use shorter names that fit in UX tooltips

CI: use shorter names that fit in UX tooltips

Change the names used for the GitHub CI workflows to be short enough
to (mostly) fit in the pop-up tool-tips that GitHub shows in the
commit view. I.e. when mouse-clicking on the passing or failing
check-mark next to the commit subject.

These names are seemingly truncated to 17-20 characters followed by
three dots ("..."). Since a "CI/PR / " prefix is added to them the job
names looked like this before (windows-test and vs-test jobs omitted):

    CI/PR / ci-config (p...
    CI/PR / windows-buil...
    CI/PR / vs-build (pu...
    CI/PR / regular (lin...
    CI/PR / regular (lin...
    CI/PR / regular (os...
    CI/PR / regular (os...
    CI/PR / regular (lin...
    CI/PR / regular (lin...
    CI/PR / dockerized (...
    CI/PR / dockerized (...
    CI/PR / dockerized (...
    CI/PR / static-anal...
    CI/PR / sparse (pu...
    CI/PR / documenta...

By omitting the "/PR" from the top-level name, and pushing the
$jobname to the front we'll now instead get:

    CI / config (push)
    CI / win build (push...
    CI / win+VS build (...
    CI / linux-clang (ub...
    CI / linux-gcc (ubun...
    CI / osx-clang (osx)...
    CI / osx-gcc (osx) (...
    CI / linux-gcc-defau...
    CI / linux-leaks (ub...
    CI / linux-musl (alp...
    CI / Linux32 (daald/...
    CI / pedantic (fedor...
    CI / static-analysis...
    CI / sparse (push)...
    CI / documentation

We then have no truncation in the expanded view. See [1] for how it
looked before, [2] for a currently visible CI run using this commit,
and [3] for the GitHub workflow syntax involved being changed here.

Let's also use the existing "pool" field as before. It's occasionally
useful to know we're running on say ubuntu v.s. fedora. The "-latest"
suffix is useful to some[4], and since it's now at the end it doesn't
hurt readability in the short view compared to saying "ubuntu" or
"macos".

1. https://github.com/git/git/tree/master/
2. https://github.com/avar/git/tree/avar/ci-rm-travis-cleanup-ci-names-3
3. https://docs.github.com/en/actions/learn-github-actions/workflow-syntax-for-github-actions
3. https://lore.kernel.org/git/d9b07ca5-b58d-a535-d25b-85d7f12e6295@github.com/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>