find: don't panic on invalid-UTF-8 arguments

[MsfPablo]

Summary

find used std::env::args(), which panics if any argument is not
valid UTF-8:

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

The panic happens during argument collection, before the -printf
format parser is ever reached.

This switches to std::env::args_os() and reports invalid UTF-8 as a
normal error (exit 1) instead of panicking:

$ find d -printf $'%\xff|\n'
find: invalid UTF-8 was found in one of the arguments: %�|
$ echo $?
1

Full binary/byte passthrough (matching GNU find's behavior of
accepting arbitrary bytes) would need find_main and the whole
argument-parsing pipeline to switch from &str to OsStr, which is
a much larger refactor — happy to look into that separately if
maintainers want it, but this fixes the immediate panic/exit-101 bug
with a minimal change.

Test plan

• Added find_printf_invalid_utf8_format_does_not_panic integration test
• cargo test — 226 + 60 + ... all passing (full suite green)
• cargo fmt --check clean
• Manually reproduced the original panic and confirmed the fix

Fixes #816

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.93%. Comparing base (1f19cdd) to head (5fe6fe6).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #817   +/-   ##
=======================================
  Coverage   91.93%   91.93%           
=======================================
  Files          35       35           
  Lines        7251     7258    +7     
  Branches      378      378           
=======================================
+ Hits         6666     6673    +7     
  Misses        443      443           
  Partials      142      142           

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[github-actions]

Commit 5fe6fe6 has test result changes:

bfs testsuite:

Test results comparison:
  Current:   TOTAL: 314 / PASSED: 267 / FAILED: 41 / SKIPPED: 6
  Reference: TOTAL: 312 / PASSED: 266 / FAILED: 40 / SKIPPED: 6

Changes from main branch:
  TOTAL: +2
  PASSED: +1
  FAILED: +1

New test failures (1):
  - gnu/okdir_path_empty

[codspeed-hq]

Merging this PR will not alter performance

✅ 20 untouched benchmarks

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_{Comparing MsfPablo:fix-printf-invalid-utf8-panic (5fe6fe6) with main (1f19cdd)}