GH Actions: Fix pip backtracking

Author: MetRonnie

.github/workflows/test.yml

@@ -65,6 +65,10 @@ jobs:
         if: github.base_ref != '8.0.x'
         uses: ./.github/actions/install-libs
         with:
+          # NOTE: these need to be manually updated as necessary:
+          cylc-flow-tag: '8.1.x'
+          # cylc-rose-tag:
+          # cylc-uis-tag:
           metomi-rose-tag: '2.0.x'
 
       - name: install eslint