ci: only upload nightlies from main

.woodpecker.yml

@@ -160,6 +160,8 @@ pipeline:
     commands:
       - git status -vvv
       - task package
+    when:
+      event: [push, tag]
 
   publish-nightly:
     image: albedo.lan:5000/upend-deploy:latest
@@ -174,6 +176,9 @@ pipeline:
       - gpg --clear-sign *.AppImage
       - scp -v *.AppImage *.asc mainsite:releases/nightly
     secrets: [gpg_sign_key, ssh_config, ssh_upload_key, ssh_known_hosts]
+    when:
+      event: push
+      branch: main
 
   publish:
     image: woodpeckerci/plugin-gitea-release