Add reading WIX_PATH environment variable

[volks73]

If the WIX_PATH environment variable exists, then this subcommand should use the value of the environment variable as the path to the WiX Toolset's bin folder, i.e. where the candle.exe and light.exe applications are located. If it is not set, then it is assumed the WiX Toolset's bin folder is in the PATH system environment variables (current behavior).

There will need to be a way for users to specify the path to the WiX Toolset's bin folder and/or compiler and linker outside of the PATH system environment variable. It is possible not everyone can modify his or her PATH or that a different version of the WiX Toolset is to be temporarily used.

[volks73]

Resolved in 0bd6517