feat(memtrack): support CODSPEED_MEMTRACK_BINARIES for exec CLI

[not-matthias]

Summary

• Add CODSPEED_MEMTRACK_BINARIES env var support in memtrack to scan additional binaries for statically linked allocators
• Auto-set the env var in the exec CLI by resolving exec target binary paths with which
• Merge with any user-provided value to allow manual override

Fixes COD-2347

Context

Memory profiling doesn't work with exec CLI when the benchmark binary has a statically linked allocator (common in Rust). Memtrack discovers allocators by scanning known build directories (target/codspeed/analysis, bazel-bin, build), but exec CLI binaries may not live there.

Test plan

• cargo check / cargo clippy clean
• cargo test passes (174 passed, 0 failed)
• Manual: codspeed exec -m memory -- ./binary_with_static_allocator — verify memtrack logs show the extra allocator

[codspeed-hq]

Merging this PR will not alter performance

✅ 4 untouched benchmarks

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_{Comparing cod-2347-clarify-spawned-process-limitation-of-memory-instrument (0abb534) with main (3e90ed6)}

[Copilot]

Pull request overview

Adds support for discovering statically linked allocators in codspeed exec targets by passing exec-resolved binaries to memtrack via a new CODSPEED_MEMTRACK_BINARIES environment variable.

Changes:

• Auto-resolve exec target binaries (via which) and set/merge CODSPEED_MEMTRACK_BINARIES in the exec CLI.
• Extend memtrack allocator discovery to scan binaries listed in CODSPEED_MEMTRACK_BINARIES.
• Add dependencies (which, temp-env) and update lockfile accordingly.

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/cli/exec/mod.rs	Resolves exec target binary paths and sets/merges CODSPEED_MEMTRACK_BINARIES before orchestrator execution.
crates/memtrack/src/allocators/mod.rs	Adds allocator discovery from env-specified binaries and wires it into AllocatorLib::find_all().
crates/memtrack/Cargo.toml	Adds temp-env as a dev-dependency (intended for tests).
Cargo.toml	Adds which dependency to support binary path resolution.
Cargo.lock	Updates dependency graph for which and adds temp-env.

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