fix build by switching to MinGW GCC 9.0

.appveyor.yml

@@ -299,7 +299,7 @@ for:
       - ps: $env:package_version = ("$(git describe --tags --always --long)").trim()
       - ps: Update-AppveyorBuild -Version "$env:package_version-$env:APPVEYOR_BUILD_NUMBER"
         before_build:
-      - cmd: if %COMPILER%==MinGW if %ARCH%==x64 set PATH=C:\mingw-w64\x86_64-8.1.0-posix-seh-rt_v6-rev0\mingw64\bin;%PATH%
+      - cmd: if %COMPILER%==MinGW if %ARCH%==x64 set PATH=C:\Qt\6.7.0\mingw_64\bin;%PATH%
       - cmd: set PATH=%QTDIR%\bin;%PATH%;C:\Qt\Tools\QtCreator\bin
       - cmd: if %COMPILER%==MinGW set PATH=%PATH:C:\Program Files\Git\usr\bin;=%
       - cmd: echo PATH=%PATH%