Use overlay

Signed-off-by: Nikolaos Karaolidis <nick@karaolidis.com>

overlays/fetchers/sshKnownHosts/default.nix

@@ -0,0 +1,33 @@
+final: prev:
+prev.lib.fetchers.withNormalizedHash { } (
+  {
+    host,
+    name ? "ssh-known-hosts-${host}",
+    outputHash,
+    outputHashAlgo,
+    port ? 22,
+    keyTypes ? [
+      "rsa"
+      "ecdsa"
+      "ed25519"
+    ],
+  }:
+  let
+    keyTypeArgs = prev.lib.concatStringsSep "," keyTypes;
+  in
+  prev.runCommandLocal name
+    {
+      inherit outputHash outputHashAlgo;
+      outputHashMode = "flat";
+      preferLocalBuild = true;
+
+      nativeBuildInputs = with final; [
+        openssh
+        gnugrep
+        coreutils
+      ];
+    }
+    ''
+      ssh-keyscan -p ${toString port} -t ${keyTypeArgs} ${host} | grep -v '^#' | sort > $out
+    ''
+)