ci: switch bench worfklow trigger in prep for rewrite

[mccutchen]

Apparently workflow_dispatch jobs are only recognized in the main branch, which is probably critical for security, but that prevents us from testing the rewrite of the bench action in #26.

See that PR for details about where this is going.

Note: this will temporarily disable the bench action.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.62%. Comparing base (8d8edfa) to head (108804f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #27   +/-   ##
=======================================
  Coverage   79.62%   79.62%           
=======================================
  Files           3        3           
  Lines         432      432           
=======================================
  Hits          344      344           
  Misses         64       64           
  Partials       24       24           

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