CI: add a 5.6 build

Add a 5.6 build to the CI.

Author: compnerd

.github/workflows/build.yml

@@ -22,6 +22,9 @@ jobs:
           - branch: swift-5.5-release
             tag: 5.5-RELEASE
 
+          - branch: swift-5.6-branch
+            tag: 5.6-DEVELOPMENT-SNAPSHOT-2022-01-11-a
+
           - branch: development
             tag: DEVELOPMENT-SNAPSHOT-2021-12-06-a