build: remove mock-fs as its broken in gituhub actions

[manchuck]

Description

Motivation and Context

Testing Details

Example Output or Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov-commenter]

Codecov Report

Merging #871 (db7a8f0) into 3.x (65d4bcf) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              3.x     #871   +/-   ##
=======================================
  Coverage   83.87%   83.87%           
=======================================
  Files         121      121           
  Lines        1619     1619           
  Branches      311      311           
=======================================
  Hits         1358     1358           
  Misses        260      260           
  Partials        1        1           

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[dragonmantank]

LGTM