Allow installing into a custom directory

[Janrupf]

Currently the installer always attempts to write to $HOME/.electron_build_tools, however, this clutters the user home directory which may be undesired if projects are located on a separate partition. Just symlinking the directory also does not work well, because the installation process doesn't work well with symlinks (and attempts to delete the symlink).

My suggestion would be to provide an environment variable ELECTRON_BUILD_TOOLS_DIR which specifies the directory the build tools should be installed to.