WIP: PLEASE, DROP THIS COMMIT BEFORE MERGE

This hook starts caching pipeline inside of the PR.
If this got merged - all PRs would start to run caching workflow (which is
undesirable, because of platform limitations)

Author: Anton-Latukha

Diff for: .github/workflows/caching.yml

@@ -42,6 +42,10 @@ on:
     # Reinstitution of the main chache puts it back into FIFO
     #   & so it gets shared across all PRs.
     - cron: "25 2/8 * * *"
+  #  2021-12-21: FIXME: Remove this endry before merge.
+  pull_request:
+    branches:
+      - '**'
 
 env:
   cabalBuild: "v2-build all --enable-tests --enable-benchmarks"