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/lib/Hydra/Controller/Build.pm

@@ -593,4 +593,12 @@ sub evals : Chained('build') PathPart('evals') Args(0) {
 }
 
 
+sub reproduce : Chained('build') PathPart('reproduce') Args(0) {
+    my ($self, $c) = @_;
+    $c->response->content_type('text/x-shellscript');
+    $c->response->header('Content-Disposition', 'attachment; filename="reproduce-build-' . $c->stash->{build}->id . '.sh"');
+    $c->stash->{template} = 'reproduce.tt';
+}
+
+
 1;