ssh/deploy-key: Skip chown if the user/group doesn't exist

This matches the behavior of NixOps.

Potential solution to #10.

src/nix/host/deploy-key.template

@@ -1,15 +1,20 @@
-
 set -euo pipefail
 
 destination=%DESTINATION%
-tmp=$destination.tmp
+tmp="${destination}.tmp"
 user=%USER%
 group=%GROUP%
 permissions=%PERMISSIONS%
 
 mkdir -p $(dirname "$destination")
 touch "$tmp"
-chown "$user:$group" $tmp
-chmod "$permissions" $tmp
+
+if getent passwd "$user" >/dev/null && getent group "$group" >/dev/null; then
+	chown "$user:$group" "$tmp"
+else
+	>&2 echo "User $user and/or group $group do not exist. Skipping chown."
+fi
+
+chmod "$permissions" "$tmp"
 cat <&0 >$tmp
 mv "$tmp" "$destination"