CI: exclude unsupported ucrt64+Skia MSYS2 matrix entry

Author: Copilot

.github/workflows/build-msys2.yml

@@ -15,6 +15,8 @@ jobs:
       matrix:
         sys: [mingw64, ucrt64, clang64]
         backend: [Cairo, Skia]
+        exclude:
+          - { sys: ucrt64, backend: Skia }
         include:
           - { sys: mingw64, icon: '🟦' }
           - { sys: ucrt64,  icon: '🟨' }