Node's existsSync() can reject runnable Store aliases because its extra
Windows-only stat check fails with EACCES (nodejs/node#36790). Use
access(F_OK) for shell validation on all platforms instead, matching the
check harness-v2's async refactor settled on (617d8b317, now pathExists
in packages/durable/src/env/node.ts). On Unix this is a no-op: existsSync
is already access(F_OK) there, so shell search order and behavior stay
unchanged; on Windows runnable aliases now pass validation.
