Allow users to reproduce builds on their own systems

You can now do:

  bash <(curl http://hydra-server/build/1238757/reproduce)

to download and execute a script that reproduces a Hydra build
locally.  This script fetches all inputs (e.g. Git repositories) and
then invokes nix-build.

The downloaded sources are stored in /tmp/build-<buildid> and reused
between invocations of the script.

Any additional command line options are passed to nix-build.  So

  bash <(curl http://hydra-server/build/1238757/reproduce) --run-env

will drop you in a shell where you can interactively hack on the
build, e.g.

  $ source $stdenv/setup
  $ set +e
  $ unpackPhase
  $ cd $sourceRoot
  $ configurePhase
  $ emacs foo.c &
  $ make

and so on.

src/root/build.tt

@@ -77,6 +77,13 @@
 
   <div id="tabs-summary" class="tab-pane active">
 
+    [% IF build.nixexprinput %]
+      <a class="btn btn-info pull-right clearfix"
+         href="[%c.uri_for('/build' build.id 'reproduce')%]">
+        Reproduce this build locally
+      </a>
+    [% END %]
+
     <table>
       <tr>
         <td>