Lock rust toolchain on v1.76.0 because higher versions segfaults during PGO gather

[MarshalX]

I don't have time to debug this shit. It just segfaults with higher rust toolchain on gha runners

refs:

• Use stable pyo3 v0.21.1 release #43
• Test PGO gather segfault #45
• Debug CI #44

and reported bug to rust repository (without repro). i will not link to to not bind issues between

[codspeed-hq]

CodSpeed Performance Report

Merging #46 will degrade performances by 26.66%

_{Comparing lock-rust-1.76.0 (d8e4386) with main (04ba564)}

Summary

⚡ 4 improvements
❌ 4 regressions
✅ 184 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	lock-rust-1.76.0	Change
❌	test_dag_cbor_decode[roundtrip05.json]	19.4 µs	25.2 µs	-23.14%
⚡	test_dag_cbor_decode[roundtrip25.json]	24 µs	18.7 µs	+28.34%
⚡	test_dag_cbor_encode[roundtrip04.json]	22.9 µs	17 µs	+34.26%
❌	test_dag_cbor_encode[roundtrip08.json]	17 µs	23.2 µs	-26.66%
⚡	test_dag_cbor_encode[roundtrip09.json]	27 µs	21.3 µs	+27.08%
⚡	test_dag_cbor_encode[roundtrip13.json]	23.1 µs	17.7 µs	+30.26%
❌	test_dag_cbor_encode[roundtrip17.json]	17.2 µs	23.1 µs	-25.37%
❌	test_dag_cbor_encode[roundtrip25.json]	16.9 µs	22.9 µs	-26.08%