CI: restore MSYS2 ucrt64 + Skia matrix job

[Copilot]

The Windows MSYS2 workflow currently supports 🚧🟨 ucrt64 (Skia), so excluding that combination was incorrect. This update restores full intended matrix coverage by removing the exclusion and running all supported MSYS2 toolchain/backend pairs.

• Workflow matrix correction

  • Updated .github/workflows/build-msys2.yml to remove the exclusion for ucrt64 + Skia.
  • Restored coverage for:
    • mingw64 + Cairo
    • mingw64 + Skia
    • ucrt64 + Cairo
    • ucrt64 + Skia
    • clang64 + Cairo
    • clang64 + Skia

• Resulting CI behavior

  • Re-enables the supported ucrt64 + Skia job.
  • Preserves full MSYS2 backend validation across all configured targets.

strategy:
  matrix:
    sys: [mingw64, ucrt64, clang64]
    backend: [Cairo, Skia]

[Kwizatz]

Excluding a whole build is not an option, specially if the combination excluded IS supported.

[Kwizatz]

Seriously? this is stupid.