run push CI only on main branch (#1343)

currently, ci runs twice for people pushing directly to this repository (e.g. https://git.deuxfleurs.fr/Deuxfleurs/garage/pulls/1340, you can see both woodpecker/{pr,pull}/debug)
that's a waste of resources, we'll do twice exactly the same thing.
i propose we only run push ci on `main-*` branches, so that creating a PR doesn't create two jobs

Reviewed-on: https://git.deuxfleurs.fr/Deuxfleurs/garage/pulls/1343
Co-authored-by: trinity-1686a <trinity@deuxfleurs.fr>
Co-committed-by: trinity-1686a <trinity@deuxfleurs.fr>

.woodpecker/debug.yaml

@@ -2,13 +2,14 @@ labels:
   nix: "enabled"
 
 when:
-  event:
-  - push
-  - tag
-  - pull_request
-  - deployment
-  - cron
-  - manual
+  - event:
+    - tag
+    - pull_request
+    - deployment
+    - cron
+    - manual
+  - event: push
+    branch: main-*
 
 steps:
   - name: check formatting