fix(ci): declare the failing-job list before the branch that can skip it

[catomean]

Follow-up to the red-base carve-out: it declared the failing-job list inside
the else branch of the "does the base have CI history?" check, but set -u is
on and the merge site always reads it.

So on a base branch with no CI history at all, the sweep took the
"proceeding" path and died:

[auto-merge] no CI history for main — proceeding
auto-merge-sweep.sh: line 249: base_red_jobs: unbound variable
exit 1

That is auto-merge dead for every PR in the repo, not a degraded path. Latent
here because this base does have CI history — but it is one fresh repo away from
being real, and it was introduced by my own change.

Fixed by declaring it once, before the branch that can skip the assignment.
Covered upstream by a regression test that runs the real script against a gh
reporting no CI history.

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