fix: remove filesystem mock, update @oclif/core to 2.8.12 (pinned) and add override

[shazron]

Motivation and Context

This is for #462 passing
The override is a workaround for #466

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:

• I have signed the Adobe Open Source CLA.
• 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]

Codecov Report

Merging #467 (5abf6a9) into master (d710269) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #467   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            5         5           
  Lines          238       238           
  Branches        47        47           
=========================================
  Hits           238       238           

Impacted Files	Coverage Δ
src/commands/discover.js	100.00% <100.00%> (ø)
src/commands/rollback.js	100.00% <100.00%> (ø)
src/commands/update.js	100.00% <100.00%> (ø)

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