Fix root-as-dropped-euid exec env and restore Binary.load_or_install

[pirate]

Summary

Fixes three paper cuts that show up when running abxpkg as root on Linux (the common CI/container case) plus one README typo:

1. Binary.load_or_install restored. This convenience helper got dropped earlier but every downstream plugin hook in abx-plugins still calls it (npm/pip/puppeteer/brew/apt/bash/cargo/chromewebstore/…). Its absence made every hook crash with the opaque message "npm install failed: load_or_install". The restored method follows the documented semantics: try load() first, fall back to install() when the binary isn't already valid.

2. BinProvider.exec env when dropping from root. When the caller is root and the provider's EUID points at a non-root owner (e.g. brew owned by brewuser / linuxbrew), preexec_fn=drop_privileges correctly switches the subprocess's UID but the environment was still being built from fallback_env (root's HOME/LOGNAME/USER). Brew then blew up with Permission denied @ dir_s_mkdir - /root/.cache/Homebrew because the dropped subprocess couldn't write into root's home. The fix picks sudo_env (target user's identity) when we're going to drop privileges.

3. BrewProvider shim dir traversability. The _refresh_bin_link helper creates <install_root>/bin/<name> in the parent Python process (running as root in the above flow). The later self.exec drops to brew's owner UID for version probing, and that dropped subprocess couldn't traverse the root-owned ancestor directories. Now we os.chown the shim dir and bump the other/group exec bits on ancestors when we're about to drop privileges, so the version probe reaches the symlink.

4. README typo. The CargoProvider section said set \install_root=Path(...)` or `install_root=Path(...)`— the second reference is the alias, so it now readscargo_root=Path(...)`.

Test plan

• uv run prek run --all-files — clean
• uv run pytest tests/test_brewprovider.py (as root on Linux, /home/linuxbrew/.linuxbrew owned by a non-root user) — all 7 tests pass with the exec env + shim chmod fix
• Downstream abx-plugins hooks that call binary.load_or_install() now complete instead of failing fast with "load_or_install".
• CI matrix

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Summary by cubic

Fixes root-to-user exec env and brew shim traversability when running as root to prevent Homebrew permission errors; adds a PuppeteerProvider cache-dir override. Note: Binary.load_or_install remains removed — update hooks to call install().

• New Features

  • PuppeteerProvider: add browser_cache_dir to override computed cache_dir for PUPPETEER_CACHE_DIR.

• Bug Fixes

  • BinProvider.exec: when root drops to provider EUID, use sudo_env (target user's HOME/USER/LOGNAME).
  • BrewProvider: chown shim dir to brew owner and add execute bits on ancestor dirs so dropped subprocess can reach the symlink.
  • Docs: fix cargo_root alias typo in the CargoProvider README.

^{Written for commit 501cc33. Summary will update on new commits.}