github: drop nu flatten in needs-more-info timer

The scheduled job has failed every night since 2026-04-13. Two
prior fixes (race-condition guards and splitting the combined
where predicate in #3200) did not address the actual cause:
`flatten` collapses nested records in a list-of-records pipeline,
so after `gh api ... | from json | flatten` the `label` and `user`
columns no longer exist - their fields are lifted to the top
level (with prefix only on naming collisions). `where label.name
== ...` and `where user.type != "Bot"` then both reference a
column that is not there.

`gh api --paginate` already returns a single concatenated JSON
array, so `from json` produces a list of records directly and no
flattening is needed. Drop both `| flatten` calls.

Verified locally with nu 0.108 against the events stream of issue
#3178: without flatten, `where event == "labeled" | where
label.name == "needs-more-info" | last` returns the labeled event
with its `label` record intact.