ci: run make check on every change and publish releases with gh

[nabil1440]

Change

• ci.yml runs make check and make release on each pull request and on each push to develop and main.
  • The repository is public. Thus, these runs do not use the organization's GitHub Actions minutes.
  • The trigger includes all pull requests, because stacked pull requests target each other and not develop.
• build.yml (the release workflow) is now one job:
  1. It makes sure that the tag commit is on main. If it is not, the release stops.
  2. It runs make check.
  3. It runs make release with the tag as VERSION.
  4. It runs gh release create with the two archives and checksums.txt. The release title is the tag.
• A tag with a pre-release suffix (for example v0.2.0-rc.1) becomes a pre-release. /releases/latest does not return a pre-release, thus installed CLIs do not update to it.
• A pre-release tag can come from any branch. Only a tag without a suffix must be on main.
• make dev-version shows <next minor version>-dev.<short commit>, for example v0.2.0-dev.1a2b3c4. make dev-release tags the commit with it and pushes the tag. Then CI publishes the pre-release.
• make dev-release refuses uncommitted changes and commits that are not pushed. It also refuses a commit if its release workflow would publish the tag as a full release.
• The workflows use actions/checkout@v7 and actions/setup-go@v7. This PR removes the archived create-release and upload-release-asset actions.
• This PR removes build.sh, because no workflow calls it now.
• The README documents CI, the release steps, dev pre-releases and how to install a dev pre-release on a test server.

Caution

CAUTION: DO NOT CHANGE THE NAMES OR THE CONTENTS OF THE RELEASE ASSETS. IF YOU CHANGE THEM, THE INSTALLED CLIS CANNOT RUN fly update.

• This PR keeps the asset names fly-linux-<arch>.tar.gz and the binary names fly-linux-<arch> in the archives.

Tests

• actionlint finds no problems in the two workflows.
• A bash test of the release steps with gh replaced by a stub:
  • A tag on origin/main passes the tag check. The top of this stack stops with the error.
  • v0.2.0 gives a release. v0.2.0-rc.1 gives a pre-release.
• gh release create --help shows each flag that the workflow uses.
• make check passes on this branch.
• make dev-version shows v0.2.0-dev.<sha>, and DEV_BASE changes the version part.
• make dev-release refuses uncommitted changes and a commit that is not pushed. The check of the release workflow refuses the lower layers of this stack and accepts this layer.
• The CI run on this pull request is the first real test of ci.yml.

The first real test of the release workflow is the next tag on main.

Closes #7