chore(ci): disable nightly channel tests to reduce Codeberg's CI usage

Turns out that our usage of the Codeberg's CI has been excessive:

https://codeberg.org/topola/topola/issues/155

The `test` flow is the longest-running (due to contracts checks), yet
the least needed, so I'm disabling it for now.

.woodpecker/test.yaml

@@ -6,8 +6,8 @@ matrix:
   include:
     - CHANNEL: stable
       FEATURES: disable_contracts
-    - CHANNEL: nightly
-      FEATURES:
+    #- CHANNEL: nightly
+      #FEATURES:
 
 steps:
   test: