only dump modules.json when it is modified

[mirpedrol]

Close #2606

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (f4cf5bd) 75.04% compared to head (b1c2d29) 75.19%.
Report is 6 commits behind head on dev.

Files	Patch %	Lines
nf_core/modules/modules_json.py	66.66%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #2609      +/-   ##
==========================================
+ Coverage   75.04%   75.19%   +0.14%     
==========================================
  Files          85       85              
  Lines        9376     9384       +8     
==========================================
+ Hits         7036     7056      +20     
+ Misses       2340     2328      -12     

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