unpack_strategy/directory: try preserving hardlinks

[cho-m]

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Try running cp -al to preserve hardlinks on both macOS and Linux.

If that fails, fall back on cp -a which can preserve hardlinks on Linux (but not macOS) assuming target filesystem supports them.

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Originally tried preserving hardlinks in #13154 but the situation with macOS commands was a bit of a pain (cp didn't work well, rsync was slow and had mtime issues, cpio was a pain to use and could lead to excessive memory as need to stdin every file, etc.).

Exploring this again to figure out a general solution for compacting bottle/install sizes in #18478. Given we now only build bottles for Ventura onward, the limitation on cp -l is less of an issue.

I think fallback on older macOS should be quick as cp will fail right away on illegal options, e.g.

❯ cp -z
cp: illegal option -- z

There is a slow path if cp -a fails and falls back to cp -pR where:

• In case of macOS, both would fail. Though I consider this similar to exception handling where this is a rare case and slowness is okay.
• In case of Linux, there is an inconvenient point where -a (implicit -R) continues on failure so it adds overhead of adding then removing files. Again should be rare situation as Linux filesystems would usually support hardlinks. Not sure if possible but maybe could create a scenario of install brew on a USB drive.

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Specific example is again trino but this time the bottle which has hardlinks:

Before:

==> Summary
🍺  /opt/homebrew/Cellar/trino/459: 6,310 files, 3.6GB
==> Running `brew cleanup trino`...
Disable this behaviour by setting HOMEBREW_NO_INSTALL_CLEANUP.
Hide these hints with HOMEBREW_NO_ENV_HINTS (see `man brew`).
==> Installation times
trino                    16.562 s

After:

==> Summary
🍺  /opt/homebrew/Cellar/trino/459: 6,310 files, 859.9MB
==> Running `brew cleanup trino`...
Disable this behaviour by setting HOMEBREW_NO_INSTALL_CLEANUP.
Hide these hints with HOMEBREW_NO_ENV_HINTS (see `man brew`).
==> Installation times
trino                    10.458 s

[MikeMcQuaid]

Thanks @cho-m! Let's try as-is and can iterate more on master.