CI: use "$runs_on_pool", not "$jobname" to select packages & config

CI: use "$runs_on_pool", not "$jobname" to select packages & config

Change the setup hooks for the CI to use "$runs_on_pool" for the
"$regular" job. Now we won't need as much boilerplate when adding new
jobs to the "regular" matrix, see 956d2e4639b (tests: add a test mode
for SANITIZE=leak, run it in CI, 2021-09-23) for the last such commit.

I.e. now instead of needing to enumerate each jobname when we select
packages we can install things depending on the pool we're running
in.

That we didn't do this dates back to the now gone dependency on Travis
CI, but even if we add a new CI target in the future this'll be easier
to port over, since we can probably treat "ubuntu-latest" as a
stand-in for some recent Linux that can run "apt" commands.

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