Set --jobs default to min(20, CPU cores)

[jwodder]

Closes #132.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.

Project coverage is 59.77%. Comparing base (2d9e853) to head (542a38e).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/main.rs	0.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #133      +/-   ##
==========================================
- Coverage   59.95%   59.77%   -0.18%     
==========================================
  Files          19       19              
  Lines        2412     2419       +7     
==========================================
  Hits         1446     1446              
- Misses        966      973       +7     

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