Admin installer fixes

[tvdeyen]

Run bundle install after adding lookbook

The following mount task will fail with a bundler error if we do not
also install the lookbook gem. Just adding it to the gemgroup
won't run bundle install for us.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 89.32%. Comparing base (41cb5d3) to head (9db13eb).
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    #5880      +/-   ##
==========================================
- Coverage   89.33%   89.32%   -0.01%     
==========================================
  Files         759      759              
  Lines       17678    17679       +1     
==========================================
  Hits        15792    15792              
- Misses       1886     1887       +1     

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[forkata]

Thanks @tvdeyen!