python: fix sysconfig.get_platform when run in a mingw shell

In a mingw shell it will call os.uname() which starts with MINGW..
resulting in bogus results. So just check for sys.platform which
is always "cygwin".

See #5143

python/900-msysize.patch

@@ -45,13 +45,13 @@
          else:
              print(cdll.LoadLibrary("libm.so"))
 --- Python-3.12.8/Lib/sysconfig.py.orig	2024-12-03 19:42:41.000000000 +0100
-+++ Python-3.12.8/Lib/sysconfig.py	2025-01-24 20:14:10.834146500 +0100
++++ Python-3.12.8/Lib/sysconfig.py	2025-02-01 17:39:28.925188300 +0100
 @@ -815,13 +815,13 @@
      elif osname[:3] == "aix":
          from _aix_support import aix_platform
          return aix_platform()
 -    elif osname[:6] == "cygwin":
-+    elif osname[:6] == "cygwin" or osname[:4] == "msys":
++    elif sys.platform == "cygwin":
 +        # https://github.com/msys2/MSYS2-packages/issues/5143
 +        # Various build tools use the platform to derive the wheel tag
 +        # and the wheel tag needs to be stable for a specific platform