Linux: Add support for snaps of CMake

Nobody has claimed this yet.

I have just installed Ubuntu 19.10 and used the Software installer application to install Visual Studio Code and CMake 3.15. I cloned my repository from GitHub and whenever I try to call CMake configure in Visual Studio code I get an error message: Bad CMake executable `/snap/bin/cmake`. CMake seems to run fine in a terminal.

VS Code/CMake extension used to run just fine out-of-the-box in previous version of Ubuntu.

When I try to enter the path manually I get error messages like these:

> [rollbar] Unhandled exception: Unhandled Promise rejection: configure TypeError: Cannot read property 'on' of undefined {}
> [rollbar] Unhandled exception: Error during CMake upgrade TypeError: Cannot read property 'on' of undefined {"version":"3.13.2","linuxURL":"https://github.com/Kitware/CMake/releases/download/v3.13.2/cmake-3.13.2-Linux-x86_64.sh","windowsURL":"https://github.com/Kitware/CMake/releases/download/v3.13.2/cmake-3.13.2-win64-x64.msi"}

Actually `/snap/bin/cmake` is not an executable but a link if I recall correctly.

Start by reproducing the configure and CMake upgrade paths in the VS Code/CMake extension on Ubuntu with `/snap/bin/cmake`, noting that it is a link rather than a regular executable. Trace how the extension validates and invokes that path; done means snap-installed CMake configures successfully without the reported errors.

Written by the indexing model from the issue text.