seq: correct fixed-width spacing for inf sequences

Pad infinity and negative infinity values with spaces when using the
`-w` option to `seq`. This corrects the behavior of `seq` to match that
of the GNU version:

    $ seq -w 1.000 inf inf | head -n 4
    1.000
      inf
      inf
      inf

Previously, it incorrectly padded with 0s instead of spaces.

tests/by-util/test_seq.rs

@@ -530,6 +530,22 @@ fn test_inf() {
     run(&["inf"], b"1\n2\n3\n");
 }
 
+#[test]
+fn test_inf_width() {
+    run(
+        &["-w", "1.000", "inf", "inf"],
+        b"1.000\n  inf\n  inf\n  inf\n",
+    );
+}
+
+#[test]
+fn test_neg_inf_width() {
+    run(
+        &["-w", "1.000", "-inf", "-inf"],
+        b"1.000\n -inf\n -inf\n -inf\n",
+    );
+}
+
 #[test]
 fn test_ignore_leading_whitespace() {
     new_ucmd!()