bug(linux): `sudo ./build.sh install` broken on current systems

[ermshiperete]

Current versions of Ubuntu and Arch Linux work with the assumption that Python packages are installed through their package manager.

If a user tries to build Keyman from source and install it with sudo ./build.sh install, the build fails with: "error: externally-managed-environment".

We should update our build script and abort with a message telling the user what to do if he encounters this situation.