- matrix: build against Qt 5 & 6

… (const char*)

fixes this error:
D:\a\qMasterPassword\qMasterPassword\src\keypress_windows.cpp(99,5): error C2664: 'void CKeyboardSimulator::TextEntry(LPCTSTR)': cannot convert argument 1 from 'const _Elem *' to 'LPCTSTR' [D:\a\_temp\build\qMasterPassword.vcxproj]

_WIN32 is defined by the compiler whereas WIN32 by the SDK.

ERROR was conflicting with a macro on Windows.

Setting CMAKE_CXX_FLAGS on the command line overrides the defaults
generated by CMake, which leads to compiler warnings and broken code. We
want to add flags to the compiler command line, hence setting CXXFLAGS
environment variable is the correct approach.

- CMakeLists.txt: prepend linker options to address linker warnings
- fix some typos
- reformat run parts to improve readability