枚举go中的字母的惯用方式是什么？

This might be the best way:

for i := 'a'; i <= 'z'; i++ {
    fmt.Println(string(i))
}

Is there a better / idiomatic approach?

for _, c := range "abcdefghijklmnopqrstuvwxyz" {
  fmt.Println(string(c))
}

This question's answers will arguably be mostly opinion-based so not really a good fit for stackoverflow, yet the way you describe is indeed acceptable as an optimal solution for the lower-case english alphabet in idiomatic Go. Other alphabets will be more complicated to formulate.

Is there a better / idiomatic approach?

No.

You can try

for i := 97 ; i < 123 ; i++ {
    fmt.Printf("%c",i)
}

Live Demo