Add CI to run every night (opensearch-project#284)

Github doesn't support non-standard like @daily, we have to use
standard cron syntax.

Signed-off-by: Vijayan Balasubramanian <balasvij@amazon.com>
(cherry picked from commit 9c64451)

.github/workflows/CI.yml

@@ -1,5 +1,7 @@
 name: Build and Test k-NN
 on:
+  schedule:
+    - cron: '0 0 * * *'  # every night
   push:
     branches:
       - "*"