don't install orjson on windows CI runners

build from source takes total CI time to 5 min (from 2) so no good

requirements.txt

@@ -14,4 +14,8 @@ ttfautohint-py==0.5.1
 cattrs==23.1.2
 # orjson currently doesn't ship 32-bit wheels for Windows
 # https://github.com/ijl/orjson/issues/409
-orjson==3.9.2; platform_python_implementation == 'CPython' and (platform_system != 'Windows' or platform_machine == 'AMD64')
+# platform_machine environment marker returns 'AMD64' on Windows even if Python is 32-bit
+# so there's no way to tell pip to install orjson only on 64-bit Win Python; hence
+# don't install orjson on Windows (works fine without it using stdlib json module)
+# https://stackoverflow.com/a/75411662
+orjson==3.9.2; platform_python_implementation == 'CPython' and platform_system != 'Windows'