xargs: strip errno when arg-file doesn't exist

[Devel08]

$ cargo run --bin xargs -- --arg-file sdf
   Compiling findutils v0.10.0 (/home/devel/Desktop/findutils)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.42s
     Running `target/debug/xargs --arg-file sdf`
Error: Failed to open sdf: No such file or directory (os error 2)

this PR gets rid of the (os error 2) in the message

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.90%. Comparing base (1f19cdd) to head (ac0171f).

Files with missing lines	Patch %	Lines
src/xargs/mod.rs	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #811      +/-   ##
==========================================
- Coverage   91.93%   91.90%   -0.03%     
==========================================
  Files          35       35              
  Lines        7251     7254       +3     
  Branches      378      378              
==========================================
+ Hits         6666     6667       +1     
- Misses        443      445       +2     
  Partials      142      142              

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[codspeed-hq]

Merging this PR will not alter performance

✅ 20 untouched benchmarks

---

_{Comparing Devel08:patch-1 (ac0171f) with main (1f19cdd)}

[github-actions]

Commit ac0171f has test result changes:

bfs testsuite:

Test results comparison:
  Current:   TOTAL: 316 / PASSED: 267 / FAILED: 43 / SKIPPED: 6
  Reference: TOTAL: 312 / PASSED: 266 / FAILED: 40 / SKIPPED: 6

Changes from main branch:
  TOTAL: +4
  PASSED: +1
  FAILED: +3

New test failures (3):
  - gnu/files0_from_ok
  - gnu/okdir_path_empty
  - gnu/okdir_path_relative