test_central_lib_dir: skip gem in post-install loop on Windows too

The inline script's gem portion is gated on Windows but the
post-script assertion loop still unconditionally expected a gem
key in the returned payload — triggering KeyError: 'gem'. Match
the script-side guard here so the loop only looks for gem when
the script ran its gem install.

Author: claude

tests/test_central_lib_dir.py

@@ -335,7 +335,10 @@ def test_real_installs_land_under_abxpkg_lib_dir(self, test_machine):
                 "bun",
                 "deno",
                 "cargo",
-                "gem",
+                # ``gem`` is only installed on non-Windows (see the
+                # platform guard in the script above + the conftest
+                # Unix-only-provider skip).
+                *(() if IS_WINDOWS else ("gem",)),
             ):
                 reported = Path(payload[provider_name])
                 assert reported == lib_dir.resolve() / provider_name, (