shuf: Do not read input when -n0 is given

[BenWiederhake]

This is explicitly tested by some suites, including the GNU test suite.

Together with #5980 and #5989, this should make uutils pass the GNU test suite for shuf. Yay! :D

[sylvestre]

please run rustfmt :)

[BenWiederhake]

Changes since last push:

• Rebased on new main
• Re-ran rustfmt, thanks @sylvestre! (I deactivated due to the rustc ICE a few days ago, and forgot to re-enable it)
• Simplified the "no output" test, thanks @cakebaker!
• Added a comment in the test, just to make extra clear that this is intentional.

[BenWiederhake]

Changes since last push:

• Added "unwritable" to the cspell-ignore header.
• test_cp::test_cp_arg_interactive_update flaked. No action taken.

[BenWiederhake]

Changes since last push:

• Rebased on current main
• Simplified a few tests by calling .no_output(). Thanks @cakebaker!
• Split test_zero_head_count_file_touch_output into three tests: Negative (tries to write to unwritable path), positive (new file created), another positive (touches existing file). Thanks @cakebaker!

[cakebaker]

Good job, thanks!