feat(walltime): search NixOS debug info path

[not-matthias]

Summary

• Fall back to /run/current-system/sw/lib/debug when /usr/lib/debug doesn't exist, so stripped system libraries resolve on NixOS hosts with environment.enableDebugInfo = true.
• Each candidate dir is checked for existence before being passed to find_debug_file_in, so missing dirs are silently skipped on either platform.

Test plan

• cargo test (existing find_debug_file_in tests still cover both build-id and .gnu_debuglink paths)
• Manual: on NixOS with environment.enableDebugInfo = true, confirm libc symbols resolve in a walltime profile

[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.

✅ 7 untouched benchmarks

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_{Comparing cod-2664-support-nixos-debug-paths-in-runner (41842da) with main (149a618)}

[GuillaumeLagrange]

lgtm