Exclude MinGW builds with Qt 6.6.0 for now

Qt 6.6.0 is currently a pre-release version, and all unit tests
are segfaulting in this specific combination at the moment.

Will revisit when Qt 6.6 is officially releases in a couple of
weeks (slated for 2023-10-10).

.github/workflows/build.yaml

@@ -287,6 +287,8 @@ jobs:
         - { arch: x86-64, qt: '5.9.9',  generator: 'MinGW Makefiles' }
         - { arch: x86-64, qt: '5.10.1', generator: 'MinGW Makefiles' }
         - { arch: x86-64, qt: '5.11.3', generator: 'MinGW Makefiles' }
+        # Exclude MinGW builds with Qt 6.6.0 for now; 6.6.0 is a pre-release, and crashing.
+        - { arch: x86-64, qt: '6.6.0', generator: 'MinGW Makefiles' }
         include: # Override default arch's for install-qt-action and msvc-dev-cmd.
         - { generator: 'MinGW Makefiles', qt: '5.12.12', qtArch: win64_mingw73 }
         - { generator: 'MinGW Makefiles', qt: '5.13.2',  qtArch: win64_mingw73 }