compiler/checker: error message differs from gc when involving builtin 'append'

[zapateo]

Source code

package main

func main() {
	type String string
	t := []byte{}
	_ = t
	s := String("ciao")
	_ = s
	_ = 2 + append(t, s...)
}

Expected error (returned by gc)

invalid operation: 2 + append(t, s...) (mismatched types int and []byte)

Error returned by Scriggo

invalid operation: 2 + append(t, s...) (cannot convert 2 (type untyped int) to type []uint8)

[gazerro]

This is a duplicate of #385.