CI: caching: build once a day

Recently there is a lot of PRs/builds from internal repo branches. Repo cache
has 10G pool limit & PR caches are not shared between PRs, but main branch cache
is.

Rebuilding it once a day ensures that if internal branch PRs pushed main state
out of the pool - it gets reinstituted.

*afaik* there is no way to prioritize main branch cache better, since GitHub CI
FIFO caches out of the pool by build time.

Author: Anton-Latukha

.github/workflows/caching.yml

@@ -30,6 +30,15 @@ on:
   push:
     branches:
       - master
+  schedule:
+    # Refresh snapshot every day at 08:25 UTC (~00:25 in California)
+    # Why: GitHub repo has 10G pool & on overflow GitHub removes caches in FIFO manner.
+    # When internal branche PRs save into the same pool -
+    #   their cache is accessible only inside of the scope of the PR.
+    # If main cache is forced out - there are no cache shared between PRs,
+    #   which implies all PRs would start to create & save their cache.
+    # Reinstitution of the main chache once a day allows to put it back so it gets shared across all PRs.
+    - cron: "25 8 * * *"
 
 env:
   cabalBuild: "v2-build all --enable-tests --enable-benchmarks"