Fixes and improvements to benchmarks

[teofr]

Some improvements to the benchmarks pipelines:

• Since V2 benchmarks are quick (~10min) and it's important not to regress in V2 it makes sense to run them on all PRs.
• Since callgrind benchmarks are so stable, we can use a window of 1, it also means after a regressing PR (that maybe we've accepted) the next PRs won't complain about it)
• Since the base sha in GH actions is problematic, we compare against the HEAD of the target branch, it's likely better.

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[github-actions]

Bencher Report

Branch	teofr/run-v2-benchmarks
Testbed	ci

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[teofr]

This PR is failing due to bencherdev/bencher#818, the fix is already merged but hasn't landed on bencher.dev

I'll mark it ready to review once that's done

[nebasuke]

Looks good, thanks!