ci: mirror release artifacts to Cloudflare R2 (#1114)

Mirrors every release artifact into Cloudflare R2 alongside the existing AWS S3
upload, so both buckets carry the same objects during a parallel period. S3 is
untouched and stays authoritative for now; once R2 is confirmed complete it can
be dropped by deleting the `blobs:` block and pointing this at R2 alone.

### Why `publishers:` and not a second `blobs:` entry

goreleaser's blob pipe authenticates from the global `AWS_*` environment and has
no per-entry credential fields, so two different credential sets (AWS S3 and
Cloudflare R2) cannot coexist in `blobs:`. Custom publishers do support
per-entry `env:`, which is the supported way to isolate the two.

### Why `curl --aws-sigv4` and not `aws`/`rclone`

The CI image `docker.gitea.com/runner-images:ubuntu-latest` ships none of `aws`,
`rclone`, `s3cmd` or `mc`, but does ship curl 8.5 with `--aws-sigv4`. This keeps
the change zero-install. Credentials are fed to curl through a config file on
stdin rather than argv, so they never appear in the process list.

### Behaviour

- Object layout is identical to S3 (`gitea-runner/<version>/<artifact>`), so
  migrating consumers later means changing only the host, not the path.
- Nightly gets R2 too, matching the existing nightly-to-S3 behaviour.
- Missing R2 configuration fails the build. Because custom publishers run as the
  very last step of the publish pipeline, a preflight `--check-config` step runs
  right after checkout so the failure happens before anything is built or
  published rather than after the release already exists.
- Verified against a local MinIO instance (site region `auto`, matching R2):
  successful upload byte-compared after download, plus the missing-variable,
  wrong-credentials, missing-file and bad-argument paths.

### Required repository secrets

These must be configured before the next release run, otherwise the new
preflight step will fail the workflow:

- `R2_ENDPOINT` (full base URL, e.g. `https://<account>.r2.cloudflarestorage.com`)
- `R2_BUCKET`
- `R2_ACCESS_KEY_ID`
- `R2_SECRET_ACCESS_KEY`

### Known, deliberate wart

The publisher runs 109 times for 73 distinct object keys: goreleaser's release
pipe already registers `release.extra_files` as `UploadableFile` artifacts, and
`internal/exec` appends the publisher's own `extra_files` on top with no
de-duplication. It cannot be globbed away because `gobwas/glob` has no
substring-exclusion matcher, so `./**.sha256` cannot be narrowed to exclude
`*.xz.sha256`. It is harmless since PUT is idempotent, and the redundant
`./**.xz` glob is kept on purpose so the publisher declares its own complete
file set instead of implicitly depending on the `release:` block's globs. This
is documented in a comment above the block.

Reviewed-on: https://gitea.com/gitea/runner/pulls/1114
Reviewed-by: Zettat123 <39446+zettat123@noreply.gitea.com>

.gitea/workflows/release-nightly.yml

@@ -20,6 +20,18 @@ jobs:
       - uses: actions/checkout@v7
         with:
           fetch-depth: 0
+      # Custom publishers (the R2 mirror below) run as the very last
+      # step of goreleaser's publish pipeline, after the Gitea release
+      # has already been created and every artifact already uploaded
+      # to S3. Fail here instead, before anything is built or
+      # published, if the R2 secrets are missing.
+      - name: check R2 configuration
+        run: sh scripts/upload-r2.sh --check-config
+        env:
+          R2_ENDPOINT: ${{ secrets.R2_ENDPOINT }}
+          R2_BUCKET: ${{ secrets.R2_BUCKET }}
+          R2_ACCESS_KEY_ID: ${{ secrets.R2_ACCESS_KEY_ID }}
+          R2_SECRET_ACCESS_KEY: ${{ secrets.R2_SECRET_ACCESS_KEY }}
       - uses: actions/setup-go@v7
         with:
           go-version-file: "go.mod"
@@ -35,6 +47,10 @@ jobs:
           AWS_SECRET_ACCESS_KEY: ${{ secrets.AWS_SECRET_ACCESS_KEY }}
           S3_REGION: ${{ secrets.AWS_REGION }}
           S3_BUCKET: ${{ secrets.AWS_BUCKET }}
+          R2_ENDPOINT: ${{ secrets.R2_ENDPOINT }}
+          R2_BUCKET: ${{ secrets.R2_BUCKET }}
+          R2_ACCESS_KEY_ID: ${{ secrets.R2_ACCESS_KEY_ID }}
+          R2_SECRET_ACCESS_KEY: ${{ secrets.R2_SECRET_ACCESS_KEY }}
           GORELEASER_FORCE_TOKEN: "gitea"
           GITEA_TOKEN: ${{ secrets.GITHUB_TOKEN }}