Add Direct URL and Delta update support to Velopack flow

[Viscerous]

Adds support for vpk direct URLs (via environment variable: VELOPACK_UPDATE_URL) and delta updates with retry behaviour.

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[Viscerous]

I have read the CLA Document and I hereby sign the CLA

[Copilot]

Pull request overview

Adds support to the viewer update flow for (1) overriding Velopack’s feed URL via VELOPACK_UPDATE_URL, and (2) pre-downloading multiple update assets to enable delta updates with a full-release fallback path.

Changes:

• Add VELOPACK_UPDATE_URL handling to bypass VVM and run Velopack update checks against a direct feed URL.
• Update Velopack asset pre-download logic from a single file to a filename→path map, enabling delta asset handling.
• Add a fallback attempt to download the full release if the delta download path fails.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
indra/newview/llvvmquery.cpp	Adds environment-variable override path to set Velopack update URL and launch an update check.
indra/newview/llvelopack.cpp	Pre-downloads multiple assets (deltas/full) and adds a fallback from delta to full release download.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Geenz]

FYI, this is on hold until we get 26.1.1 out the door.