core: persist /usr/local/bin via profile.d helper

[MickLesk]

✍️ Description

What does this PR do?

• Replaces inline PATH manipulation in setup_uv() with a new helper function ensure_usr_local_bin_persist().
• This function ensures that /usr/local/bin is included in the $PATH, both for the current shell session and persistently via /etc/profile.d/custom_path.sh.

Why?

• In some environments (especially containers), /usr/local/bin is not included in $PATH by default.
• The previous solution appended the path at runtime, but did not persist it across logins or reboots.
• This change makes uv and other binaries reliably executable without requiring re-export or login shell restarts.

Additional considerations

• The function checks if pveversion exists and skips changes on Proxmox nodes to avoid affecting system-level behavior.

✅ Prerequisites (X in brackets)

• Self-review completed – Code follows project standards.
• Tested thoroughly – Changes work as expected.
• No security risks – No hardcoded secrets, unnecessary privilege escalations, or permission issues.

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🛠️ Type of Change (X in brackets)

• 🐞 Bug fix – Resolves an issue without breaking functionality.
• ✨ New feature – Adds new, non-breaking functionality.
• 💥 Breaking change – Alters existing functionality in a way that may require updates.
• 🆕 New script – A fully functional and tested script or script set.
• 🌍 Website update – Changes to website-related JSON files or metadata.
• 🔧 Refactoring / Code Cleanup – Improves readability or maintainability without changing functionality.
• 📝 Documentation update – Changes to README, AppName.md, CONTRIBUTING.md, or other docs.