Use compute credits for macOS PRs

.cirrus.yml

@@ -26,6 +26,11 @@ env:
   # Compile and run the tests
   EXAMPLES: yes
 
+compute_credits_template: &CREDITS_TEMPLATE
+  # https://cirrus-ci.org/pricing/#compute-credits
+  # Only use credits for pull requests to the main repo
+  use_compute_credits: $CIRRUS_REPO_FULL_NAME == 'bitcoin-core/secp256k1' && $CIRRUS_PR != ""
+
 cat_logs_snippet: &CAT_LOGS
   always:
     cat_tests_log_script:
@@ -128,6 +133,7 @@ task:
   brew_script:
     - brew install automake libtool gcc
   << : *MERGE_BASE
+  << : *CREDITS_TEMPLATE
   test_script:
     - ./ci/cirrus.sh
   << : *CAT_LOGS