Fix sandbox lookbook installation

[AlistairNorman]

Summary

bin/sandbox is erroring right now. We just need a slightly different command because the bundle when installing lookbook fails. This fixes #6416

Checklist

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.50%. Comparing base (1db0767) to head (0db9d44).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...erators/solidus_admin/install/install_generator.rb	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6417   +/-   ##
=======================================
  Coverage   89.50%   89.50%           
=======================================
  Files         981      981           
  Lines       20479    20479           
=======================================
  Hits        18330    18330           
  Misses       2149     2149           

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