only serialize the pythonjob data when launching

[superstar54]

To avoid load profile when visualizing the wg in the widget, we should not serialize the data when run wg.to_dict(), instead, we serialize the data only when we want to launch the workgraph.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 89.47368% with 2 lines in your changes missing coverage. Please review.

Project coverage is 80.70%. Comparing base (5937b88) to head (49df54e).
Report is 71 commits behind head on main.

Files with missing lines	Patch %	Lines
aiida_workgraph/tasks/pythonjob.py	87.50%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #309      +/-   ##
==========================================
+ Coverage   75.75%   80.70%   +4.94%     
==========================================
  Files          70       66       -4     
  Lines        4615     5083     +468     
==========================================
+ Hits         3496     4102     +606     
+ Misses       1119      981     -138     

Flag	Coverage Δ
python-3.11	80.62% <89.47%> (+4.95%)	⬆️
python-3.12	80.62% <89.47%> (?)
python-3.9	80.65% <89.47%> (+4.92%)	⬆️

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