fix: only strip trailing os error suffix

[xtqqczze]

Closes #13723

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/misc/io-errors (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/misc/tty-eof (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/seq/seq-epipe is now being skipped but was previously passing.
Congrats! The gnu test tests/tail/tail-n0f is now passing!

[sylvestre]

how often this happens ?
i already gave my opinion here:
#13723

[xtqqczze]

how often this happens ?

Er, I don't know how often this happens. But it still seems like a good idea to ensure that a function documented to "strip the trailing (os error XX)" only strips the trailing occurrence, rather than an occurrence in the middle.

[oech3]

I prefer previous code for simplicity until someone find actial issue.

[sylvestre]

same, sorry