syncthing: remove runtime dependency on go

The hack used here is shamelessly stolen from buildGoPackage. If it is
going to be applied to more expressions, it may make sense to factor it
out so it can be shared.

pkgs/applications/networking/syncthing/default.nix

@@ -1,4 +1,10 @@
 { stdenv, lib, fetchFromGitHub, go, pkgs }:
+let
+  removeExpr = ref: ''
+    sed -i "s,${ref},$(echo "${ref}" | sed "s,$NIX_STORE/[^-]*,$NIX_STORE/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee,"),g" \
+  '';
+
+in
 
 stdenv.mkDerivation rec {
   version = "0.14.18";
@@ -42,6 +48,10 @@ stdenv.mkDerivation rec {
                --replace /usr/bin/syncthing $out/bin/syncthing
   '';
 
+  preFixup = ''
+    find $out/bin -type f -exec ${removeExpr go} '{}' '+'
+  '';
+
   meta = with stdenv.lib; {
     homepage = https://www.syncthing.net/;
     description = "Open Source Continuous File Synchronization";