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/tail/tail-n0f (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/resolution (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/symlink (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/cp/link-heap is now being skipped but was previously passing.
Note: The gnu test tests/cut/bounded-memory is now being skipped but was previously passing.

[codspeed-hq]

Merging this PR will not alter performance

✅ 319 untouched benchmarks
⏩ 46 skipped benchmarks¹

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_{Comparing puneetdixit200:fix/12407-install-stdin-existing-target (d288a3a) with main (3af25ae)²}

Footnotes

1. 46 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on main (d41c56b) during the generation of this report, so 3af25ae was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[puneetdixit200]

I checked the new red CI on this PR. The diff only touches install code/tests (src/uu/install/src/install.rs, tests/by-util/test_install.rs, and tests/by-util/test_mkfifo.rs). The current failures are in unrelated targets: fuzz jobs for date, dirname, echo, non_utf8_paths, parse_time, and split, plus test_stdbuf_no_fork_regression.\n\nThe fuzz_date failure also matches the open upstream issue #12458. I do not see a PR-scoped code change to make for these failures.