Use PosixPath to parse a filename rather than a regexp

[fyellin]

FIX #1352

• Fixes #XXX
• Were any Django, import pipeline, or table_schema files modified? B
• Were any JavaScript or CSS files modified? N
• Was the documentation reviewed for necessary changes or additions? NA Description of changes:

Rewrite property volume_set to use PosixPath rather than a regexp. Code was failing because

1. The regexp was bogus. [/$] doesn't mean what I intended it to mean
2. The regexp couldn't handle hyphens and periods in the directory name

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (a421cb8) to head (66c4b95). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1357 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 41 41 Lines 25001 25001 Branches 1217 1181 -36 ========================================= Hits 25001 25001 ```

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