ci: publish the attestation bundle with each release

[groupsky]

Each release now also has nodkey_<version>.sigstore.json: the build provenance attestation of both tarballs, as one Sigstore bundle. With this file, gh attestation verify --bundle needs no GitHub login. Without it, gh needs a login even for this public repository (cli/cli#11803). A broker host should have no GitHub login. The installer and nodkey update (#88) will use this file.

Changes

• .github/workflows/release.yml, publish job:
  • The attest step has an id. A new step copies its bundle-path file to dist/nodkey_<version>.sigstore.json.
  • A new step checks both tarballs with this bundle and no login, with the same command as the docs. If the check fails, the release stays a draft.
  • The upload step adds the bundle to the files, before the release is published.
  • "Check the files and the tag" now also checks that the tag is v<version>. The version names the bundle file, and nobody can change a published release.
  • No new permission and no new action. Values reach the shell only through env:.
• docs/releasing.md:
  • It lists the files of a release.
  • "Verify a download" uses VERSION and ARCH variables, shows a curl download with no login, and shows the --bundle check. It also says to use gh 2.93.0 or newer. No command line ends in a # comment: interactive zsh does not treat it as a comment by default.
  • A new step 3 checks the version. The attestation proves which workflow built the file, but not which version it is. Step 3 checks that the file's own name is in the attestation. This works on any machine. Then nodkey version on the machine the binary is for.
• DESIGN.md section 18 has one sentence on the bundle file.

Review

The security review found nothing high or medium. These points are fixed:

• gh release download needs a login only in gh older than 2.96.0. The docs said it always does.
• Step 3, the version check, is new. Tested with no login: the 0.1.0 tarball passes it; the same file named nodkey_9.9.9_linux_amd64.tar.gz fails it.
• The publish job checks that TAG matches VERSION.
• The check step prints gh --version in the log.
• The commands in the docs use one style.

The reviewer tested the check with gh 2.96.0 and no login. Both 0.1.0 tarballs passed. These all failed: an empty file, {}, a cut bundle, two bundles in one file, a release attestation, the wrong file and a missing bundle.

The last box of #87 (a real release has the file, and the check passes with no login) gets ticked after the next release, so this pull request does not close the issue.

Part of #87.