Windows build fails if VS150COMNTOOLS is set

If a developer tries to override the default Visual Studio root dir (e.g., to speficy Community rather than Professional), the build will fail.

To reproduce this, set the VS150COMNTOOLS env variable to the same value that it will use if not set (note that the double backslash is needed when running from cygwin shell).

$ export VS150COMNTOOLS="C:\\Program Files (x86)\\Microsoft Visual Studio\\2017\\Professional\\VC\\Auxiliary\\Build"
$ gradle
...
:buildSrc:build
\Microsoft was unexpected at this time.

FAILURE: Build failed with an exception.

* Where:
Script 'C:\Users\kcr\javafx\10-kcr\jfx\rt\buildSrc\win.gradle' line: 48

* What went wrong:
A problem occurred evaluating script.
> Process 'command 'cmd'' finished with non-zero exit value 255

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.

* Get more help at https://help.gradle.org

BUILD FAILED in 10s