Glance: preserve glance.yml across updates

[michelroegl-brunner]

✍️ Description

Glance updates used CLEAN_INSTALL=1, which removed /opt/glance/glance.yml while redeploying the prebuild binary only. The service then failed because systemd points at the missing config file.

This change backs up glance.yml to /tmp before the clean install and restores it afterward, matching patterns used by other scripts (e.g. Homer, rustypaste).

🔗 Related Issue

Fixes #14826

✅ Prerequisites (X in brackets)

• Self-review completed – Code follows project standards.
• Tested thoroughly – Changes work as expected. (Requires a Glance LXC; see test plan below.)
• 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 script metadata (PocketBase/website data).
• 🔧 Refactoring / Code Cleanup – Improves readability or maintainability without changing functionality.
• 📝 Documentation update – Changes to README, AppName.md, CONTRIBUTING.md, or other docs.