Build release artifacts and SLSA provenance in CI

Adds .github/workflows/release.yml, triggered on `release: created`, which:
  - checks out the release's target_commitish,
  - builds the source zip via CMake/CPack (`package_source`),
  - uploads the zip to the draft release,
  - calls slsa-framework/slsa-github-generator to produce a
    SLSA v1.0 *.intoto.jsonl provenance file and attach it to the
    same draft release.

After CI completes, the draft has both the zip and the provenance attached,
and the maintainer reviews and publishes as before.

Updates support/release.py to stop building and uploading the zip locally;
that work has moved to CI so the SLSA provenance attests to the actual
build environment that produced the artifact, not to a hash observed
after the fact. The script still builds docs locally because the
subsequent mkdocs deploy step depends on them.

support/release.py

@@ -151,12 +151,17 @@ if __name__ == '__main__':
     fmt_repo.add(changelog)
     fmt_repo.commit('-m', 'Update version')
 
-    # Build the docs and package.
+    # Build the docs locally; the source zip is now built and attached to the
+    # release in CI by .github/workflows/release.yml, which also generates a
+    # SLSA provenance attestation for it.
     run = Runner(fmt_repo.dir)
     run('cmake', '.')
-    run('make', 'doc', 'package_source')
+    run('make', 'doc')
 
-    # Create a release on GitHub.
+    # Create a draft release on GitHub. The release workflow triggers on
+    # `release: created`, builds the source zip from `target_commitish`, and
+    # attaches the zip plus *.intoto.jsonl provenance to this draft. After
+    # reviewing the draft, the maintainer clicks Publish to finalize.
     fmt_repo.push('origin', 'release')
     auth_headers = {'Authorization': 'token ' + os.getenv('FMT_TOKEN')}
     req = urllib.request.Request(
@@ -169,20 +174,6 @@ if __name__ == '__main__':
         if response.status != 201:
             raise Exception(f'Failed to create a release ' +
                             '{response.status} {response.reason}')
-        response_data = json.loads(response.read().decode('utf-8'))
-        id = response_data['id']
-
-    # Upload the package.
-    uploads_url = 'https://uploads.github.com/repos/fmtlib/fmt/releases'
-    package = 'fmt-{}.zip'.format(version)
-    req = urllib.request.Request(
-        f'{uploads_url}/{id}/assets?name={package}',
-        headers={'Content-Type': 'application/zip'} | auth_headers,
-        data=open('build/fmt/' + package, 'rb').read(), method='POST')
-    with urllib.request.urlopen(req) as response:
-        if response.status != 201:
-            raise Exception(f'Failed to upload an asset '
-                            '{response.status} {response.reason}')
 
     short_version = '.'.join(version.split('.')[:-1])
     check_call(['./mkdocs', 'deploy', short_version])