explain: Switch default to global plan instead of physical plan

[def-]

Was forgotten in #31185

3c1c409 changes the default for a bare EXPLAIN to use EXPLAIN PHYSICAL PLAN. But I only changed the default in order to catch every bare EXPLAIN, and I change the default back in c97eb41.

@mgree This is a good argument for at least having one test remaining that has a bare EXPLAIN I guess ;)

Noticed by @ggevay in https://materializeinc.slack.com/archives/C08ACQNGSQK/p1739461427061339

Checklist

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