v.util: use tmp instead of cache dir for temporary diff files (#21075)

Author: ttytm

vlib/v/util/diff/diff.v

@@ -84,14 +84,16 @@ pub fn color_compare_files(diff_cmd string, file1 string, file2 string) string {
 }
 
 pub fn color_compare_strings(diff_cmd string, unique_prefix string, expected string, found string) string {
-	cdir := os.join_path_single(os.cache_dir(), unique_prefix)
-	os.mkdir(cdir) or {}
+	tmp_dir := os.join_path_single(os.vtmp_dir(), unique_prefix)
+	os.mkdir(tmp_dir) or {}
+	defer {
+		os.rmdir_all(tmp_dir) or {}
+	}
 	ctime := time.sys_mono_now()
-	e_file := os.join_path_single(cdir, '${ctime}.expected.txt')
-	f_file := os.join_path_single(cdir, '${ctime}.found.txt')
+	e_file := os.join_path_single(tmp_dir, '${ctime}.expected.txt')
+	f_file := os.join_path_single(tmp_dir, '${ctime}.found.txt')
 	os.write_file(e_file, expected) or { panic(err) }
 	os.write_file(f_file, found) or { panic(err) }
 	res := color_compare_files(diff_cmd, e_file, f_file)
-	os.rmdir_all(cdir) or {}
 	return res
 }