pinky: fix panic when outputting to /dev/full

[hideyosh1]

• fixes panic by replacing all println with writeln

new contributor, please be gentle

Closes #10562

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/cut/bounded-memory is now being skipped but was previously passing.
Congrats! The gnu test tests/rm/many-dir-entries-vs-OOM is now passing!

[oech3]

  error: empty string literal in `writeln!`
     --> src/uu/pinky/src/platform/unix.rs:235:9
      |
  235 |         writeln!(stdout().lock(), "")?;
      |         ^^^^^^^^^^^^^^^^^^^^^^^^----^
      |                                 |
      |                                 help: remove the empty string

[codspeed-hq]

Merging this PR will not alter performance

✅ 289 untouched benchmarks
⏩ 66 skipped benchmarks¹

---

_{Comparing hideyosh1:pinky-full (94bc512) with main (52e1bc1)}

Footnotes

1. 66 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩