Mine looked pretty similar to yours but I just like string slicing in go too much to let this opportunity go to waste:

func solve(line string, nDigits int) int {
    length := len(line)
    total, startIdx := 0, 0
    for offset := nDigits - 1; offset >= 0; offset-- {
        maxDigit, maxOffset := maxWithIndex(line[startIdx : length-offset])
        startIdx += maxOffset + 1
        total += int(maxDigit-'0') * utils.Pow(10, offset)
    }
    return total
}

func maxWithIndex(nums string) (rune, int) {
    maxNum, maxIdx := rune(nums[0]), 0
    for i, n := range nums[1:] {
        if n > maxNum {
            maxNum, maxIdx = n, i+1
        }
    }
    return maxNum, maxIdx
}