Fix output_rows_skew sqllogictest flake

[geoffreyclaude]

Which issue does this PR close?

• None.

Rationale for this change

The output_rows_skew sqllogictest can fail nondeterministically after #21351 added dynamic FileStream work scheduling: reorderable sibling file scan streams may share one queue of unopened files. The failing case expects the four files to be attributed to output partitions as [4, 0, 1, 0], producing output_rows_skew=84.31%; CI has observed the same scan reported as 100% instead.

This was seen on PR #21927 and independently on main, so it is a pre-existing test flake.

What changes are included in this PR?

Adds WITH ORDER (x) to the single CREATE EXTERNAL TABLE skew_parquet statement used by the four-file case in explain_analyze.slt. That makes the scan order-preserving, which disables shared file work stealing for this assertion and keeps the expected per-partition metric deterministic.

Are these changes tested?

Yes: flaky test now passes consistently.

Are there any user-facing changes?

No.

[adriangb]

I wonder if we wouldn't be better off accepting that scan orders are going to be mostly non-deterministic and either add an ORDER BY to the queries themselves or classify this metric via #21160

[geoffreyclaude]

I wonder if we wouldn't be better off accepting that scan orders are going to be mostly non-deterministic and either add an ORDER BY to the queries themselves or classify this metric via #21160

@adriangb I'll admit I didn't look too much in detail at this, my Codex just randomly fixed it while working on another task. So if you have a cleaner fix please go ahead!