harcode paths for kpppoed, add port num in config

pkgs/go-l2tp/default.nix

@@ -1,6 +1,7 @@
 {
   buildGoModule
 , fetchFromGitHub
+, pppBuild
 }:
 
 buildGoModule rec {
@@ -13,6 +14,13 @@ buildGoModule rec {
     rev = "570d763";
     hash= "sha256-R8ImKPkPBC+FvzKOBEZ3VxQ12dEjtfRa7AH94xMsAGA=";
   };
+
+  patchPhase = ''
+    sed -i.bak -e 's:/usr/sbin/pppd:${pppBuild}/bin/pppd:' cmd/kl2tpd/pppd.go
+    sed -i.bak -e 's:/usr/sbin/kl2tpd:${placeholder "out"}/bin/kl2tpd:' cmd/kpppoed/l2tpd_kl2tpd.go
+    grep bin/kl2tp cmd/kpppoed/l2tpd_kl2tpd.go
+  '';
+
   doCheck = false;
   vendorHash = "sha256-hOkhJhToN/VJwjQmnQJSPGz26/YDR2Ch+1yeW51OF+U=";