fix: Update relative path calculation

[ianlewis]

Description:

Updates the relative path calculation so that the issue-reopener action will work with checkouts outside the GITHUB_WORKSPACE.

Related Issues:

Fixes #262

Checklist:

• Review the CONTRIBUTING.md documentation.
• Add a reference to a related issue in the repository.
• Add a description of the changes proposed in the pull request.
• Sign the Google CLA.
• Commits include a Developer Certificate of Origin signoff.

  This is my commit message
  Signed-off-by: Random J Developer <random@developer.example.org>
  

• Unit tests

[codecov]

Codecov Report

Merging #491 (0e366a4) into main (9a7c862) will increase coverage by 0.01%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #491      +/-   ##
==========================================
+ Coverage   80.30%   80.32%   +0.01%     
==========================================
  Files          19       19              
  Lines        1097     1098       +1     
  Branches       14       14              
==========================================
+ Hits          881      882       +1     
  Misses        155      155              
  Partials       61       61              

Files Changed	Coverage Δ
actions/issue-reopener/src/action.ts	90.90% <100.00%> (ø)
actions/issue-reopener/src/reopener.ts	100.00% <100.00%> (ø)