fix(test): avoid powershell flake by spawning absolute-path cmd.exe (closes #95)

[zackees]

Summary

• test_process_command on Windows was spawning powershell by basename, relying on SearchPathW resolution during Command::spawn. Under concurrent workspace test load that occasionally failed with ERROR_FILE_NOT_FOUND (surfaced as "failed to launch QEMU at powershell: program not found").
• Switch to %SystemRoot%\System32\cmd.exe with an absolute path and cmd /C "echo line1& echo line2". echo is a cmd built-in so there is no nested executable lookup, and the absolute path bypasses PATH races entirely.
• Matches the request to reduce dependence on PowerShell across the test harness where cmd-level functionality suffices.

Test plan

• uv run cargo test -p fbuild-daemon --lib handlers::emulator run 5× consecutively — 0 flakes.
• Per-run wall-clock dropped from ~0.54 s to ~0.16 s.
• uv run cargo clippy --workspace --all-targets -- -D warnings clean.
• Full uv run cargo test --workspace — the specific regression (run_avr8js_headless_captures_stdout) is gone. (An unrelated flake in toolchain::esp_qemu::hydrate_windows_runtime_copies_iconv_next_to_exe was observed once under the same concurrent load and passes in isolation — tracked separately; same root cause class as perf(build): investigate warm-pass compilation stall — 30s where cache says <1s #91.)

Related

• Closes flake(test): Windows emulator tests spawn 'powershell' by basename, fail under concurrent load #95
• Same class of Windows Command::spawn issue uncovered during perf(build): investigate warm-pass compilation stall — 30s where cache says <1s #91 warm-build profiling.

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• crates/fbuild-daemon/src/handlers/emulator.rs

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• Create PR with unit tests
• Commit unit tests in branch fix/95-test-process-command-cmd-exe

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