cmake: pass QT_QML_GENERATE_QMLLS_INI via environment variable

Barebones editors like neovim can't set QT_QML_GENERATE_QMLLS_INI variables to CMake (they don't have access the CMake).
Find a way to communicate "QT_QML_GENERATE_QMLLS_INI" to CMake for these bare bones editors.
One option could be to read an environment variable from CMake, using the https://cmake.org/cmake/help/latest/command/configure_file.html#configure-file mechanism to force reconfigurations on environment variable content change.