[InterNetNews/inn] 93b15d: Add GitHub Actions configuration

[Russ Allbery]

  Branch: refs/heads/ci
  Home:   https://github.com/InterNetNews/inn
  Commit: 93b15d51292c1ad9be3a4492cc6f94eca70b40b8
      https://github.com/InterNetNews/inn/commit/93b15d51292c1ad9be3a4492cc6f94eca70b40b8
  Author: Russ Allbery <eagle at eyrie.org>
  Date:   2021-09-12 (Sun, 12 Sep 2021)

  Changed paths:
    A .github/dependabot.yml
    A .github/workflows/build.yaml
    M MANIFEST
    A ci/README.md
    A ci/apt-packages
    A ci/install
    A ci/test

  Log Message:
  -----------
  Add GitHub Actions configuration

Configure a build workflow running on pushes and pull requests
that does the same build logic as support/mksnapshot does, except
for testing --enable-largefiles (which is irrelevant on GitHub
Actions runners).

Tell dependabot to send PRs for updates to any workflow actions
we use (currently only checkout).