fix: detect tools correctly on Linux

[sumant-suresh]

Summary

Fixes Linux tool detection in caveman doctor.

On Linux, command is a shell builtin and cannot be executed via spawnSync(). As a result, tool detection incorrectly reports installed tools such as git, tar, and curl as missing.

Example before:

[fail] Tool: git — missing
[fail] Tool: tar — missing
[fail] Tool: curl — missing
Even when all binaries are available on PATH.

Verified on Ubuntu 20.04:

git, tar, and curl available in /usr/bin
node can resolve all tools successfully
caveman doctor reports the tools correctly after the change