Add a warning when the CLI is used but without the keyword indicating if it is being installed or similar

Add a warning when the CLI is used but without the keyword indicating if it is being installed or similar. For example if it is started like:

qt-unified-installer.run --accept-licenses

then it will seemingly accept the command arguments but they will have no effect since the install keyword is missing. Adding a warning here would be good to make it clearer that it might be missing.