builtin: add a temporary ctovstring_impl/1 API to enable ui to compile cleanly for PR#25264, part 1

vlib/builtin/string.v

@@ -3052,3 +3052,16 @@ pub fn (mut ri RunesIterator) next() ?rune {
 	}
 	return rune(impl_utf8_to_utf32(start, len))
 }
+
+// ctovstring_impl is a temporary API, to enable clean CI runs for https://github.com/vlang/v/pull/25264 .
+// It will be deleted after the migration to the new `builtin` naming scheme is finished.
+@[export: 'builtin__ctovstring']
+pub fn ctovstring_impl(s &u8) string {
+	unsafe {
+		len := C.strlen(voidptr(s))
+		return string{
+			str: memdup(voidptr(s), isize(len))
+			len: len
+		}
+	}
+}