ci: sign release checksums with the imported GPG key (#1382)

The release job has imported the Teabot GPG key since https://gitea.com/gitea/runner/pulls/116, but `.goreleaser.yaml` never had a `signs:` block, so releases were never signed. Add it so `checksums.txt` gets a detached `checksums.txt.sig`, the same way https://gitea.com/gitea/cli signs its releases.

The nightly job imports the key as well since the `signs:` block applies to nightly builds, and the R2 publisher mirrors the signature to dl.gitea.com.

_Written by Claude (Opus 5.5)._

---------

Co-authored-by: Lunny Xiao <[email protected]>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1382
Reviewed-by: Lunny Xiao <[email protected]>

.goreleaser.yaml

@@ -80,6 +80,7 @@ builds:
 publishers:
   - name: cloudflare-r2
     checksum: true
+    signature: true
     extra_files:
       - glob: ./**.xz
       - glob: ./**.sha256
@@ -100,6 +101,10 @@ checksum:
   extra_files:
       - glob: ./**.xz
 
+signs:
+  - artifacts: checksum
+    args: ["--batch", "-u", "{{ .Env.GPG_FINGERPRINT }}", "--output", "${signature}", "--detach-sign", "${artifact}"]
+
 snapshot:
   version_template: "{{ .Branch }}-devel"