make getFileSize file system independent

[alexkli]

Follow up from #113.

For the git comparator numbers for measuring the checkout folder size are different between CircleCI, GH actions and local Mac in the unit tests (local branch for example) even though the contents of the folders are identical. This might be because of different Linux versions and/or file systems.

Compare

• https://app.circleci.com/pipelines/github/adobe/sizewatcher/319/workflows/25d54edb-292d-452a-af16-cc4dcf226fdf/jobs/315
• https://github.com/adobe/sizewatcher/actions/runs/14213116331/job/39823823685?pr=113