More explainable benchmark GHA.

[trexfeathers]

Closes #6513 , closes #6514

If this seems over-detailed: the difference between Iris and Iris-esmf-regrid has historically caused confusion and slowed us down.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.89%. Comparing base (ecb558a) to head (5a70d6d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6556   +/-   ##
=======================================
  Coverage   89.89%   89.89%           
=======================================
  Files          90       90           
  Lines       24138    24138           
  Branches     4492     4492           
=======================================
  Hits        21699    21699           
  Misses       1679     1679           
  Partials      760      760           

☔ 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.