util: make permission walks symlink-safe

[erneestoc]

Problem

The recursive permission walk set_perms_recursive_impl (which drives both
set_readonly_recursive and set_dir_writable_recursive) used fs::metadata
(stat), which follows symlinks. On input trees containing symlinks — e.g.
.venv/bin/python3 produced by rules_python / rules_apple venv tooling —
this has two failure modes:

• A symlink to a directory reports is_dir() == true, so the walk recurses
  through the link, escaping the materialized tree or descending into an
  unrelated directory.
• A symlink is passed to set_permissions; chmod follows symlinks, so it
  mutates the link's target. When the target does not exist (a dangling link
  — common when a venv points outside the action's input set), the chmod
  returns ENOENT and fails the entire walk.

That ENOENT failure surfaces as set_readonly_recursive erroring inside
DirectoryCache::get_or_create, which makes prepare_action_inputs log
"Directory cache failed, falling back to traditional download" and take the
slow download_to_directory path.

Fix

set_perms_recursive_impl now uses symlink_metadata (lstat) and returns
early on symlink entries — it never chmods a symlink and never recurses
through one. Regular files keep their existing read-only (0o555) treatment,
so the CAS-hardlinked-inode hermeticity contract (#2347) is unchanged.

hardlink_directory_tree_recursive already recreated symlinks as symlinks; its
symlink branch is reordered ahead of the is_dir() / is_file() branches to
make the symlink-first intent explicit and robust.

Testing

Adds regression tests covering the set-readonly, set-dir-writable, and
hardlink/clone walks over a tree containing a symlink to an in-tree file, a
dangling relative symlink, and a symlink to an in-tree directory — asserting
each walk succeeds and the symlinks are preserved with their targets intact.

bazel test //nativelink-util/... //nativelink-worker/... — 27/27 pass;
clippy + rustfmt aspects clean.

Note

Companion to #2357 (worker: make directory-cache entries already-writable).
Both PRs touch nativelink-util/src/fs_util.rs
(hardlink_directory_tree_recursive); whichever lands second needs a trivial
rebase.

🤖 Generated with Claude Code

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This change is 

[MarcusSorealheis]

LGTM by itself