ci: skip arm if no worker is configured

.cirrus.yml

@@ -24,6 +24,9 @@ env:  # Global defaults
 # by installing qemu-user-static, which works out of the box with
 # podman or docker. Background: https://stackoverflow.com/a/72890225/313633
 #
+# Alternatively arm tasks can be skipped by setting NO_ARM=1 as a custom env
+# variable in Cirrus.
+#
 # The above machine types are matched to each task by their label. Refer to the
 # Cirrus CI docs for more details.
 #
@@ -126,7 +129,9 @@ task:
 
 task:
   name: 'ARM, unit tests, no functional tests'
+  # Skip if no ARM worker exists (for a fork)
   << : *GLOBAL_TASK_TEMPLATE
+  skip: $NO_ARM == "true"
   persistent_worker:
     labels:
       type: arm64  # Use arm64 worker to sidestep qemu and avoid a slow CI: https://github.com/bitcoin/bitcoin/pull/28087#issuecomment-1649399453