Fix worktree counting on Julia 1.13

[tecosaur]

Rather trivial. Fixes #63.

Can confirm that all tests, and Polyester.jl tests, now pass on 1.13-rc1.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.20%. Comparing base (cb9cf25) to head (6a78229).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
src/threadtasks.jl	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #64      +/-   ##
==========================================
- Coverage   99.20%   95.20%   -4.01%     
==========================================
  Files           4        4              
  Lines         126      125       -1     
==========================================
- Hits          125      119       -6     
- Misses          1        6       +5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[tecosaur]

The invalidations check itself seems broken, rather than anything to do with this change.