Build fails with Go 1.26 rc1

[stefanb]

👋 Build problem found downstream in Homebrew (I am a maintainer):

• go 1.26rc2, go@1.25 1.25.6 (new formula) Homebrew/homebrew-core#258912

Confirmed locally via simple go build command inside this repository:

$ go1.26rc1 build ./...
# vitess.io/vitess/go/hack
go/hack/ensure_swiss_map.go:39:38: invalid array length -1 (untyped int constant)

The Swiss-maps-ensuring Go build flag:

vitess/go/hack/ensure_swiss_map.go

Line 1 in b3666b2

//go:build !goexperiment.swissmap

is no longer good enough for Go 1.26, as in Go 1.26 and later Swiss maps are enabled by default and there is no more GOEXPERIMENT=noswissmap nor GOEXPERIMENT=swissmap flag either.

See:

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