Add "ExtraSearchPaths=" and --extra-search-paths

Add "ExtraSearchPaths=" and --extra-search-paths

This variable keeps a colon-delimited list of paths to be prepended to
PATH in the context of mkosi execution. Unlike shell variable
expansion, if any path refers to an unset variable, that path will be
ignored.

Besides the environment variables, the variable SUDO_HOME can be used
to refer to the home directory of the user calling mkosi with
sudo. This allows settings like

    ExtraSearchPaths=$SUDO_HOME/go/bin