Allows environment variables when defining the version path

In some project we use a "Environment_XXX" folder that contains all libraries needed for building it, also Qt. The folder itself is versioned because libraries can change during the development, calling "Environment_XXX_1_0", "Environment_XXX_1_2" and so on. We define the folder using an environment variable, so when we update it, the solution will always find the correct libraries since it refers to them by using the environment variable. With the plugin this is not possible, since the path of a Qt version is always absolute and we cannot use environment variables in the path, so every time that we change the folder we need to manually update the version in the plugin.

Please allow the plugin to use environment variables when setting Qt version paths, so we can use the same env. variable that we use elsewhere and the update of environment will be transparent.