/etc/ssh/ssh_known_hosts: refactor and fix #5612

Generating the file was refactored to be completely in nix.
Functionally it should create the same content as before,
only adding the newlines.

CC recent updaters: @aszlig, @rickynils.

nixos/modules/services/networking/ssh/sshd.nix

@@ -17,13 +17,11 @@ let
 
   knownHosts = map (h: getAttr h cfg.knownHosts) (attrNames cfg.knownHosts);
 
-  knownHostsFile = pkgs.runCommand "ssh_known_hosts" {} ''
-    touch "$out"
-    ${flip concatMapStrings knownHosts (h: ''
-      pubkeyfile=${builtins.toFile "host.pub" (if h.publicKey == null then readFile h.publicKeyFile else h.publicKey)}
-      ${pkgs.gnused}/bin/sed 's/^/${concatStringsSep "," h.hostNames} /' $pubkeyfile >> "$out"
-    '')}
-  '';
+  knownHostsText = flip (concatMapStringsSep "\n") knownHosts
+    (h:
+      concatStringsSep "," h.hostNames + " "
+      + (if h.publicKey != null then h.publicKey else readFile h.publicKeyFile)
+    );
 
   userOptions = {
 
@@ -301,7 +299,7 @@ in
       { source = "${cfgc.package}/etc/ssh/moduli";
         target = "ssh/moduli";
       }
-      { source = knownHostsFile;
+      { text = knownHostsText;
         target = "ssh/ssh_known_hosts";
       }
     ];