fix[tool]: fix combined_json output for CLI

[charles-cooper]

What I did

fix #3900

How I did it

How to verify it

Commit message

the output json would not be produced because Path does not have a json
serializer

there are actually tests for `combined_json`, but they test the
`compile_files` API directly, whereas the offending code is in the very
outer `_cli_helper()` function.

the best (long-term) way to test this might be to have a harness which
runs the vyper CLI directly from shell, but that is not explored here
to reduce scope.

Description for the changelog

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[codecov-commenter]

Codecov Report

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

Project coverage is 59.26%. Comparing base (5bdd174) to head (2d8978c).
Report is 1 commits behind head on master.

❗ Current head 2d8978c differs from pull request most recent head 3011847. Consider uploading reports for the commit 3011847 to get more accurate results

Files	Patch %	Lines
vyper/cli/vyper_compile.py	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##           master    #3901       +/-   ##
===========================================
- Coverage   86.33%   59.26%   -27.08%     
===========================================
  Files          92       92               
  Lines       14036    14036               
  Branches     3085     3085               
===========================================
- Hits        12118     8318     -3800     
- Misses       1490     5090     +3600     
- Partials      428      628      +200     

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