Fix swapfile "insecure permissions"

Kristian Klausen requested to merge klausenbusk/arch-boxes:swap-perms into master

This isn't really necessary as /swap is mode 0700, but a warning is logged in the journal[1], so let's set /swap/swapfile to mode 0600 and get rid of the warning.

[1] swapon[189]: swapon: /swap/swapfile: insecure permissions 0644, 0600 suggested.

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