Intro: Windows: ticking "add to path" in ARM gdb install overwrites user's PATH

[MirkoCovizzi]

After following the guide here https://github.com/rust-embedded/book/blob/master/src/intro/install/windows.md I sadly discovered that my user PATH variables were overwritten by the latest ARM install. My suggestion is to not specify (or suggest) to tick the option to add the program to PATH until the bug is sorted out in the ARM installer.