ci: unify setup of some environment variables

ci: unify setup of some environment variables

Both GitHub Actions and Azure Pipelines set up the environment variables
GIT_TEST_OPTS, GIT_PROVE_OPTS and MAKEFLAGS. And while most values are
actually the same, the setup is completely duplicate. With the upcoming
support for GitLab CI this duplication would only extend even further.

Unify the setup of those environment variables so that only the uncommon
parts are separated. While at it, we also perform some additional small
improvements:

    - We now always pass `--state=failed,slow,save` via GIT_PROVE_OPTS.
      It doesn't hurt on platforms where we don't persist the state, so
      this further reduces boilerplate.

    - When running on Windows systems we set `--no-chain-lint` and
      `--no-bin-wrappers`. Interestingly though, we did so _after_
      already having exported the respective environment variables.

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