Disable admin preview for extensions test apps

[tvdeyen]

Summary

The new admin is far from complete and existing
extensions will fail if we test them with the
new default admin that we generate apps with.

It can be enabled by setting ADMIN_PREVIEW=true
in your test env.

Checklist

Check out our PR guidelines for more details.

The following are mandatory for all PRs:

• I have written a thorough PR description.
• I have kept my commits small and atomic.
• I have used clear, explanatory commit messages.

The following are not always needed:

• 📖 I have updated the README to account for my changes.
• 📑 I have documented new code with YARD.
• 🛣️ I have opened a PR to update the guides.
• ✅ I have added automated tests to cover my changes.
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[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (be710f3) 88.58% compared to head (27b94a9) 88.57%.

Files	Patch %	Lines
core/lib/spree/testing_support/common_rake.rb	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5600      +/-   ##
==========================================
- Coverage   88.58%   88.57%   -0.01%     
==========================================
  Files         685      685              
  Lines       16404    16405       +1     
==========================================
  Hits        14531    14531              
- Misses       1873     1874       +1     

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[github-actions]

💚 All backports created successfully

Status	Branch	Result
✅	v4.3

Questions ?

Please refer to the Backport tool documentation and see the Github Action logs for details