seq: Do not allow -w and -f to be specified at the same time

Fixes #7466.

tests/by-util/test_seq.rs

@@ -19,6 +19,14 @@ fn test_no_args() {
         .stderr_contains("missing operand");
 }
 
+#[test]
+fn test_format_and_equal_width() {
+    new_ucmd!()
+        .args(&["-w", "-f", "%f", "1"])
+        .fails_with_code(1)
+        .stderr_contains("format string may not be specified");
+}
+
 #[test]
 fn test_hex_rejects_sign_after_identifier() {
     new_ucmd!()