Fix using run with file descriptors

[Bobronium]

• I have added a news fragment under changelog.d/ (if the patch affects the end users)

Fixes #1292

Summary of changes

Add app_path.is_fifo() check and read it out.

Test plan

Tested by running

python -m pipx run <(echo 'print("Hello world!")')

[Bobronium]

Hmm. I just noticed that maybe_script_content now might return Path unchanged on main as opposed to 1.4.3 where it always read out content.

Have to do things differently then. Checking additionally for app_path.is_fifo() doesn't work right away, need more debugging to figure out why.

Upd. wasn't able to preserve fd for passing it as script path later. Reading content for these seems like a sane middle ground, and it works \o/

[chrysle]

Please add an example for this functionality to docs/examples.md.