Constify fmt::from_fn

[EFanZh]

See #117729 (comment).

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[GKFX]

Is this going to be on its own feature flag (which I think will require its own tracking issue)? Although it's a minor change, I would expect it's too late to include it in fmt_from_fn as that would need to be backported into the currrent beta (1.93).

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