node-pty declares the spawn-helper target inside binding.gyp's OS=="mac"
block and pty.cc execs it only under __APPLE__. Asserting it on
`!== 'win32'` made every Linux orcad boot degraded with
spawn_helper_missing while its terminals worked fine.
Route all four sites through one shared `usesNodePtySpawnHelper`
predicate: the precondition verdict, the prebuilt slot install, the
+x repair, and the prebuilds build script (which threw outright on a
Linux slot build).
Fixes#17844
