Support Go 1.26 and later with Swiss maps always enabled

[stefanb]

Description

Adjusts Go build flags for ensuring Swiss maps are available to not trigger false positive compile-time errors with Go 1.26 and later.
In Go 1.26 and later Swiss maps are always enabled and cannot be disabled anymore (as was the case in Go 1.24-1.25) and there is no more GOEXPERIMENT=noswissmap flag. See:

• golang/go@2ae059c
• runtime: use SwissTable golang/go#54766

Related Issue(s)

Fixes #19087

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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Deployment Notes

N/A

AI Disclosure

No AI was used for preparing this patch.

[vitess-bot]

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[dbussink]

We should flip this flag to goexperiment.swissmap .noswissmap instead of !goexperiment.swissmap which would work also still for 1.26 if the flag is entirely gone?

[stefanb]

Yes, that would be much cleaner, if goexperiment.noswissmap build constraint existed.

GOEXPERIMENT=noswissmap environment setting existed in Go 1.24 and 1.25 to disable swiss maps and was negated mapped to goexperiment.swissmap build constraint. Hence the confusion.

In go 1.26 GOEXPERIMENT=noswissmap is not supported anymore and there is no way to set goexperiment.swissmap build constraint either, so the only option is to check for go1.26 build constraint.

[mattlord]

Thanks, @stefanb !