When Nushell is already installed in the OS, complete-update should use
the system binary instead of requiring a compiled binary first. This
removes the circular dependency:
BEFORE: complete-update required compiled nu binary
- If nu not compiled: ❌ error
- Force-compiles to 0.108.0 even if 0.109 update requested
AFTER: complete-update now has smart fallback
- Try compiled binary first (if exists)
- Fall back to system nu (if available)
- Only error if neither exists
This makes the script work in real-world scenarios where users have
Nushell installed in their OS.
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