logstash: fix bin path

with the last commit for logstash plugin the bin path was not being used and the executables
were written directly in the root directory
this results in the failure of the logstash service configuration.
additionally the logstash tool itself does not start because it cannot source shell libraries relative
to the current location

pkgs/tools/misc/logstash/default.nix

@@ -16,9 +16,8 @@ stdenv.mkDerivation rec {
 
   installPhase = ''
     mkdir -p $out
-    cp -r {Gemfile*,vendor,lib} $out
+    cp -r {Gemfile*,vendor,lib,bin} $out
-    cp bin/logstash $out/logstash
+    mv $out/bin/plugin $out/bin/logstash-plugin
-    cp bin/plugin $out/logstash-plugin
   '';
 
   meta = with stdenv.lib; {