don't install pytest-codspeed on non x86_64 machines

[davidhewitt]

Change Summary

Should fix failures like https://github.com/pydantic/pydantic-core/actions/runs/5879887441/job/15946120306

Related issue number

N/A

Checklist

• Unit tests for the changes exist
• Documentation reflects the changes where applicable
• Pydantic tests pass with this pydantic-core (except for expected changes)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

Selected Reviewer: @adriangb

[codecov]

Codecov Report

Merging #895 (b597ca4) into main (e56491e) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #895   +/-   ##
=======================================
  Coverage   93.74%   93.74%           
=======================================
  Files         105      105           
  Lines       15333    15333           
  Branches       25       25           
=======================================
  Hits        14374    14374           
  Misses        953      953           
  Partials        6        6           

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[codspeed-hq]

CodSpeed Performance Report

Merging #895 will improve performances by 28.51%

_{Comparing dh/codspeed-limits (b597ca4) with main (e56491e)}

Summary

🔥 1 improvements
✅ 137 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	dh/codspeed-limits	Change
🔥	test_dont_raise_error	38.9 µs	30.3 µs	+28.51%

[davidhewitt]

please review