Follow XDG spec for sst cli install location

[arockett]

Instead of saving the cli tool to ~/.sst/bin, use the XDG standard $HOME/.local/bin.

Even though the XDG Base Directory Specification does not specify an env var or default location for the "single base directory relative to which user-specific executable files may be written", the community dicussion seems to have settled on $XDG_BIN_HOME with default to $HOME/.local/bin and various OS projects and installers have adopted these values already.