openssh.authorizedKeys.keyFiles: allow multiple keys
openssh.authorizedKeys.keyFiles: allow multiple keys

Ugly hack to get around the error "a string that refers to a store
path cannot be appended to a path".  The underlying problem is that
you cannot do

  "${./file1} ${./file2}"

but you can do

  " ${./file1} ${./file2}"

Obviously we should allow the first case as well.

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