add note about why this change to the actions config is necessary

Author: cosmicexplorer

.github/workflows/continuous-integration.yml

@@ -12,6 +12,9 @@ jobs:
       matrix:
         operating-system: [ubuntu-latest, macos-latest, windows-latest]
         node-version: [12.x, 14.x, 16.x, 18.x, 20.x]
+        # The 'setup-node' action has become unable to locate versions for macos ("darwin") on arm64
+        # for some reason. Processor architecture shouldn't affect any of our testing, so we can pin
+        # this for now.
         architecture: [x64]
 
     steps: