Switch to walltime runner for benchmarks

[seddonym]

Moves to Codspeed's walltime runner for benchmarks, which is much faster than the CPU-based one.

I needed to move grimp out of my personal Github account to an organization, to get this working.

[codspeed-hq]

CodSpeed Performance Report

Merging #256 will degrade performances by 69.13%

_{Comparing switch-to-walltime (ffe8efa) with main (e239dcd)}

Summary

⚡ 1 improvement
❌ 2 (👁 2) regressions
✅ 18 untouched
🆕 2 new
⏩ 23 skipped¹

Benchmarks breakdown

	Mode	Benchmark	BASE	HEAD	Change
👁	WallTime	test_chain_found	15.3 µs	17 µs	-10.11%
👁	WallTime	test_chains_found	27.4 µs	88.9 µs	-69.13%
🆕	WallTime	test_chains_found_sparse_imports	N/A	222.6 µs	N/A
🆕	WallTime	test_build_django_from_cache_a_few_misses[350]	N/A	260.8 ms	N/A
⚡	WallTime	test_build_django_uncached	4,436.2 ms	151.3 ms	×29

Footnotes

1. 23 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩