clippy(cp): fix collapsible_if lint

[mattsu2020]

Summary

• apply clippy::collapsible_if fixes in cp.rs and copydir.rs
• replace nested conditionals with let chains where applicable
• keep behavior unchanged while simplifying the control flow

Testing

• cargo +stable test -p uu_cp --lib

How it was applied

cargo +stable clippy -p uu_cp --fix --all-targets --all-features -- -A clippy::all -W clippy::collapsible_if
cargo +stable fmt --all -- --emit files

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/pr/bounded-memory (fails in this run but passes 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/cut/cut-huge-range is now passing!

[cakebaker]

Thanks!