bug#64074: guix [COMMAND] --load-path does not check if path is valid

[Christian Miller]

User can set load path with -L or --load-path in commands like guix build, guix time-machine and so on.  But this flag does not check if the path is actually a valid directory.  You can point to a directory that doesn't even exist on the filesystem.  It should do that just like the -f or --file flag for the guilx build command.