benchmark: make child_process tests windows-compatible

[ri7116]

The spawn-echo and exec-stdout benchmarks fail on Windows due to use of Unix-specific commands.
Replace Unix-specific 'echo' command with 'cmd.exe /c echo' on Windows to ensure cross-platform compatibility.

[nodejs-github-bot]

Review requested:

• @nodejs/performance

[richardlau]

The spawn-echo and exec-stdout benchmarks fail on Windows due to use of Unix-specific commands. Replace Unix-specific 'echo' command with 'cmd.exe /c echo' on Windows to ensure cross-platform compatibility.

FWIW For Windows, we state in BUILDING.md that Unix tools are a pre-req for running tests:

node/BUILDING.md

Lines 647 to 649 in 91f035e

	* Basic Unix tools required for some tests,
	[Git for Windows](https://git-scm.com/download/win) includes Git Bash
	and tools which can be included in the global `PATH`.

cc @nodejs/platform-windows

[StefanStojanovic]

I agree with the previous comment. These changes are unnecessary as echo should be available on Windows if BUILDING.md is followed correctly.