hooks/shutdown: copy /. instead of /

cp will fail to copy `/` if the target directory already exists.
The produced error is not very helpful:

    cp: cannot create directory '/run/initramfs/': No such file or directory

Instead of copying `/`, copy `/.` to avoid the issue.

Fixes #104,
Fixes https://bugs.archlinux.org/task/74534
1 job for fix-copy-in-shutdown in 11 seconds (queued for 1 second)