Enable benchmarking to run on `main`

[trexfeathers]

Fixes an oversight where Cirrus' benchmarking script depended entirely on $CIRRUS_BASE_SHA, which only exists on pull requests.

[codecov[bot]]

Codecov Report

Merging #226 (c9b0efd) into main (f0781aa) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #226   +/-   ##
=======================================
  Coverage   99.36%   99.36%           
=======================================
  Files          28       28           
  Lines        2829     2829           
=======================================
  Hits         2811     2811           
  Misses         18       18           

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[trexfeathers]

✔

[trexfeathers]

Thanks @stephenworsley !