ci: do not run CI on fork branch push

.cirrus.yml

@@ -48,7 +48,12 @@ env:  # Global defaults
 
 # https://cirrus-ci.org/guide/tips-and-tricks/#sharing-configuration-between-tasks
 filter_template: &FILTER_TEMPLATE
-  skip: $CIRRUS_REPO_FULL_NAME == "bitcoin-core/gui" && $CIRRUS_PR == ""  # No need to run on the read-only mirror, unless it is a PR. https://cirrus-ci.org/guide/writing-tasks/#conditional-task-execution
+  # No need to run on the read-only mirror, unless it is a PR.
+  # https://cirrus-ci.org/guide/writing-tasks/#conditional-task-execution
+  skip: $CIRRUS_REPO_FULL_NAME == "bitcoin-core/gui" && $CIRRUS_PR == ""
+  # Do not run branch pushes on forks, because they are typically redundant with
+  # pull requests to the main repo.
+  skip: $CIRRUS_REPO_OWER != "bitcoin" && $CIRRUS_REPO_OWER != "bitcoin-core" && $CIRRUS_PR == ""
   stateful: false  # https://cirrus-ci.org/guide/writing-tasks/#stateful-tasks
 
 base_template: &BASE_TEMPLATE

.github/workflows/ci.yml

@@ -9,7 +9,10 @@ on:
   # See: https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#push.
   push:
     branches:
-      - '**'
+      # Do not run branch pushes on forks, because they are typically redundant with
+      # pull requests to the main repo.
+      - 'bitcoin/**'
+      - 'bitcoin-core/**'
     tags-ignore:
       - '**'