install: handle piped stdin when target exists

[puneetdixit200]

Fixes #12407.

This updates the copy path to compare the opened source file metadata with the destination instead of canonicalizing the source path. That keeps openable special paths such as /dev/fd/0 working when the destination already exists while preserving same-file detection.

Tests:

• cargo fmt --check
• cargo test --features install --no-default-features test_install::test_install_from_stdin -- --nocapture
• cargo test --features install --no-default-features test_install:: -- --nocapture
• cargo test -p uu_install
• cargo clippy --features install --no-default-features --all-targets -- -D warnings

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/cut/cut-huge-range is now passing!