Run nightly 10 times (#694)

We think #690 fixed our
nightly issues. At the very least, we cannot reproduce it locally
anymore. However, we've also seen that nightly fails more often than
local runs, so we'll run nightly a bunch of times in the coming days to
see if we've actually fixed things.

.github/workflows/ci.yml

@@ -15,7 +15,16 @@ on:
     # GitHub scheduling. See the comment at 'branches-ignore' for more
     # information on our development flow. This job will run in context of the
     # branch 'staging'.
-    - cron:  '0 19 * * *'
+    - cron:  '0  19 * * *'
+    - cron:  '30 19 * * *'
+    - cron:  '0  20 * * *'
+    - cron:  '30 20 * * *'
+    - cron:  '0  21 * * *'
+    - cron:  '30 21 * * *'
+    - cron:  '0  22 * * *'
+    - cron:  '30 22 * * *'
+    - cron:  '0  23 * * *'
+    - cron:  '30 23 * * *'
 
   push:
       branches-ignore: