Only use backslash when setting PATH and other related enviroment variables in Windows with Powershell or cmd

[traversaro]

As reported by @S-Dafarra in a internal issue, the default installation instructions of the robotology-superbuild do not work correctly, because the PATH is populate by file paths with the forward slashes, while the PATH env variable apparently supports only backslash:

While this is a critical issue, it went unnoticed for the following reason:

• I (and I guess some other users) also on Windows use the Git Bash, and source the environment using the setup.sh scripts, that due to the conversions in the PATH env variable between the Git enviroment and the Windows enviroment are not affected by this problem.
• Most of the executables worked fine as they were in the same directory of the .dll they used, the only library that were affected by this were the YARP plugins in bin/YARP, bin/iCub or similar.