feat(runner): wire CoreCLR symbolization for walltime .NET runs

[not-matthias]

Summary

• enable Samply's CoreCLR support for walltime profiles
• preserve existing runtime environment values through Samply's unset-only behavior
• make managed .NET JIT frames available for symbolization

Verification

• cargo check -p codspeed-runner
• repository pre-commit checks, including fmt and clippy

[codspeed-hq]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 17 untouched benchmarks

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_{Comparing csharp-support (1222358) with main (37eba9e)}