Maybe this

.woodpecker/.build.yml

@@ -21,7 +21,7 @@
                 "nix develop --command dotnet test --configuration Release"
             ],
             "when": [
-                { "event": "push", "evaluate": "CI_COMMIT_BRANCH == CI_REPO_DEFAULT_BRANCH" },
+                { "evaluate": "CI_PIPELINE_EVENT == \"push\" && CI_COMMIT_BRANCH == CI_REPO_DEFAULT_BRANCH" },
                 { "event": "pull_request" }
             ]
         }