sort: strip the errno suffix from read errors

[MsfPablo]

Fixes #13992.

sort /proc/self/mem prints:

sort: Input/output error (os error 5)

The SortError variants all run their io errors through strip_errno already; this one path does not. read_to_buffer in chunks.rs builds a USimpleError straight from e.to_string(), so the errno tail survives. Switched it to strip_errno, which is the only remaining to_string on an io error in the crate.

Tests: a unit test driving read_to_buffer with a reader that fails with EIO. It reproduces the reported message exactly on the old code and passes on the new one.

[sylvestre]

please only add a test in test_sort.rs

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/date/resolution (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/csplit/csplit-heap is now being skipped but was previously passing.
Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.