homepage: Add build time var

[burgerga]

✍️ Description

Minor update: Makes sure that the date reported on the homepage matches the published date on the release:

(it is possible to to have the commit hash instead of "source" but without graphql api that is a bit annoying, see https://stackoverflow.com/questions/67040794/how-can-i-get-the-commit-hash-of-the-latest-release-from-github-api. At least this is a step in the right direction)

The jq installation code for the update script is mostly copied from misc/tools.func (replaced return with exit).

🔗 Related PR / Issue

Link: #

✅ 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.