fix(preprocess): using iota outside constant declaration

[Villaquiranm]

closes #5668

[Gno2D2]

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[davd-gzl]

Not directly related. Declaring a variable name iota := 0 is legal in Go (https://go.dev/play/p/FP9F1VlxwAt), and is now returning "cannot use iota outside constant declaration".

Either we should copy Go behavior, or set builtin identifiers cannot be shadowed: iota. like it is done for const iota = 5