main/postmarketos-base-ui: add "$HOME/.local/bin" to $PATH
This directory is specified by the XDG Base Directory specification (https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html). Quoting:
User-specific executable files may be stored in $HOME/.local/bin. Distributions should ensure this directory shows up in the UNIX $PATH environment variable, at an appropriate place.
So let's actually do this. This way also binaries installed by e.g. Pip or Cargo will be usable out-of-the-box rather than that the user has to figure out why it's not appearing in their PATH.