find: avoid panic on a non-UTF-8 argument

[AlejandroCoronadoN]

Summary

A non-UTF-8 argument makes find panic during argument collection, before the format
parser is ever reached:

$ find d -printf $'%\xff|\n'
thread 'main' panicked at ...: called `Result::unwrap()` on an `Err` value
$ echo $?
101

Root cause

main collected arguments with std::env::args(), which panics on a non-UTF-8 argument.

Fix

Collect with std::env::args_os() and report the first non-UTF-8 argument as an error,
exiting with code 1 instead of aborting. The conversion is extracted into a small
collect_utf8_args helper so it can be unit-tested. Full GNU-style byte pass-through of
the format string is a larger change and left as a follow-up.

Verification

$ find d -printf $'%\xff|\n'
find: invalid (non-UTF-8) argument: %<0xff>|
$ echo $?
1

Adds unit tests collects_valid_utf8_args and non_utf8_arg_is_rejected_not_panicking.
cargo fmt and cargo clippy --bin find are clean.

Fixes #816.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.17%. Comparing base (2a3eac9) to head (5103940).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #833      +/-   ##
==========================================
+ Coverage   92.15%   92.17%   +0.01%     
==========================================
  Files          35       35              
  Lines        7377     7395      +18     
  Branches      383      383              
==========================================
+ Hits         6798     6816      +18     
  Misses        438      438              
  Partials      141      141              

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[codspeed-hq]

Merging this PR will not alter performance

✅ 20 untouched benchmarks

---

_{Comparing AlejandroCoronadoN:fix-find-non-utf8-args (5103940) with main (2a3eac9)}

[github-actions]

Commit 2e69802 has test result changes:

bfs testsuite:

Test results comparison:
  Current:   TOTAL: 312 / PASSED: 266 / FAILED: 40 / SKIPPED: 6
  Reference: TOTAL: 313 / PASSED: 267 / FAILED: 40 / SKIPPED: 6

Changes from main branch:
  TOTAL: -1
  PASSED: -1
  FAILED: +0

[sylvestre]

please add a test_find.rs

[AlejandroCoronadoN]

Done, added an integration test in test_find.rs that passes a non-UTF-8 argument and checks it errors cleanly instead of panicking.

[github-actions]

Commit 977aa58 has test result changes:

bfs testsuite:

Test results comparison:
  Current:   TOTAL: 315 / PASSED: 267 / FAILED: 42 / SKIPPED: 6
  Reference: TOTAL: 314 / PASSED: 266 / FAILED: 42 / SKIPPED: 6

Changes from main branch:
  TOTAL: +1
  PASSED: +1
  FAILED: +0

New test failures (1):
  - gnu/okdir_path_dot

Test improvements (1):
  + gnu/files0_from_ok