Function user_executable_directory 

pub fn user_executable_directory(
    override_variable: Option<&'static str>,
) -> Option<PathBuf>

Returns an appropriate user-level directory for storing executables.

This follows, in order:

• $OVERRIDE_VARIABLE (if provided)
• $XDG_BIN_HOME
• $XDG_DATA_HOME/../bin
• $HOME/.local/bin

On all platforms.

Returns None if a directory cannot be found, i.e., if $HOME cannot be resolved. Does not check if the directory exists.