builtin: fix the string.index_any method (#11310)

vlib/builtin/string.v

@@ -739,12 +739,12 @@ fn (s string) index_kmp(p string) int {
 
 // index_any returns the position of any of the characters in the input string - if found.
 pub fn (s string) index_any(chars string) int {
+	for i, ss in s {
 		for c in chars {
-		idx := s.index_(c.ascii_str())
-		if idx == -1 {
-			continue
+			if c == ss {
+				return i
+			}
 		}
-		return idx
 	}
 	return -1
 }

vlib/builtin/string_test.v

@@ -910,3 +910,9 @@ fn test_string_to_rune() {
 	x := 'Hello World 👋'
 	assert x.runes().len == 13
 }
+
+fn test_index_any() {
+	x := 'abcdefghij'
+	assert x.index_any('ef') == 4
+	assert x.index_any('fe') == 4
+}