add codeberg ci and cleanup github one (#311)

- apt-get is slow, takes up ~1m40s just to install deps, fix it by not
  using it. instead use alpine linux for codeberg ci, which brings build
  time down to 25s.
- And since alpine uses musl, it's probably a good idea to use it on our
  ci since it might catch us using any glibc extensions. The github ci
  can keep using ubuntu.
- remove duplication of CFLAGS by having it on a separate file instead.
- remove pull_request from github ci since we no longer accept PRs
  there.

Closes: https://codeberg.org/nsxiv/nsxiv/issues/307

Reviewed-on: https://codeberg.org/nsxiv/nsxiv/pulls/311
Reviewed-by: Berke Kocaoğlu <berke.kocaoglu@metu.edu.tr>
Reviewed-by: TAAPArthur <taaparthur@noreply.codeberg.org>

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