Only attempt loading ESM bundles from URL when running server

[philippjfr]

The recent PR that added support for loading ESM bundles from a server endpoint did not consider that in the notebook this wouldn't be possible. This PR ensures that we only attempt to do so when actually running on the server.

[codecov]

Codecov Report

Attention: Patch coverage is 95.83333% with 1 line in your changes missing coverage. Please review.

Project coverage is 82.23%. Comparing base (f621f0c) to head (3df8756).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
panel/custom.py	95.83%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7412      +/-   ##
==========================================
+ Coverage   82.22%   82.23%   +0.01%     
==========================================
  Files         337      337              
  Lines       50572    50582      +10     
==========================================
+ Hits        41583    41597      +14     
+ Misses       8989     8985       -4     

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