This PR updates the Linux install script to support installing a
specific version of Zed via the
ZED_VERSION environment variable.

- If ZED_VERSION is set, that version will be downloaded.
- If ZED_VERSION is not set, the script defaults to the latest stable
version, preserving
  existing behavior.
- Works for both Linux and macOS installs.
- Maintains compatibility with ZED_CHANNEL for preview builds.

This enhancement allows users to:
- Pin a specific Zed version for reproducible setups or CI workflows.
- Easily install older or known-good versions without manually
downloading release assets.

Usage examples:
- Install latest stable (default):
  curl -f https://zed.dev/install.sh | sh
- Install specific version:
  curl -f https://zed.dev/install.sh | ZED_VERSION=0.216.0 sh
  
  Use this — it’s correct, concise, and clearly user-facing:

Release Notes

* **Added:** Support for installing a specific Zed version via the
`ZED_VERSION` environment variable in the install script (defaults to
`latest` when unset).