Merge #20328: cirrus: Skip tasks on the gui repo main branch

66667ac cirrus: Skip tasks on the gui repo main branch (MarcoFalke)

Pull request description:

  No need to run every build twice, once in the main repo and then in the read-only gui mirror repo

ACKs for top commit:
  decryp2kanon:
    ACK 66667ac
  hebasto:
    ACK 66667ac, though still preferring `only_if` as showing skipped tasks as successful ones seems a bit confused.

Tree-SHA512: 0d35bd115152e06ba4dc5f364130ba5496167d960c44eac2c76192ff9bf7c51f46ab72e2d054dcc6a91818a18dffbbc262f8a4c4483857158c0af4f55dfe9b28

Author: MarcoFalke

.cirrus.yml

@@ -19,6 +19,7 @@ env:
 
 # https://cirrus-ci.org/guide/tips-and-tricks/#sharing-configuration-between-tasks
 global_task_template: &GLOBAL_TASK_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
   ccache_cache:
     folder: "/tmp/ccache_dir"
   depends_built_cache: