Don't run CodSpeed on merge_group events

[hynek]

It's not supported.

[hynek]

@webknjaz can you quickly rubber-stamp this, so I can test the merge queue?

ref: https://github.com/python-attrs/attrs/actions/runs/9975150415/job/27564200157#step:5:68

[codspeed-hq]

CodSpeed Performance Report

Merging #1305 will not alter performance

_{Comparing cod-merge (0010b45) with main (3df6017)}

Summary

✅ 12 untouched benchmarks

[webknjaz]

@hynek sorry, I was AFK but looks reasonable. Technically, if this was like super desired, the workflow could be migrated to the push event since Merge Queues also push to a temporary branch, so two events are emitted and some services don't really care enough about separating them.