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

[Christian Miller via Bug reports for GNU Guix]

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.