Operation EnvironmentVariable clears environment

The EnvironmentVariable operation in Qt Installer Framework component scripting API is supposed to set an environment variable - which it does. However, when it's used for the first time, it also clears the environment.

STEPS TO REPRODUCE:
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1) Add 'component.addOperation("EnvironmentVariable", "SFDK_AUTO_STOP_VMS", "1")'
to createOperations() in one of the components
2) Build the installer
3) Run the installer

EXPECTED RESULT:
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The Execute operations after the EnvironmentVariable operation are run with modified environment, so that the SFDK_AUTO_STOP_VMS environment variable is added to the system enviroment

ACTUAL RESULT:
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Only SFDK_AUTO_STOP_VMS variable is set, otherwise the environment is empty

I have already a fix for the bug, I'll prepare a pull request shortly.