GitHub Action : Use year, month, and day as cache timestamp key

Original example for timestamp included hour, minute and second to build up cache key.

.github/workflows/ResInsightWithCache.yml

@@ -66,7 +66,7 @@ jobs:
         id: cache_timestamp_string
         shell: cmake -P {0}
         run: |
-          string(TIMESTAMP current_date "%Y-%m-%d-%H;%M;%S" UTC)
+          string(TIMESTAMP current_date "%Y-%m-%d" UTC)
           message("::set-output name=timestamp::${current_date}")
       - name: Cache Buildcache
         id: cache-buildcache