nixos/tests/letsencrypt: Fix go source install

Since e95f17e2

, Go packages no longer
contain the source tree, however Boulder seems to need that as it
generates a few files during build.

Ideally we would only pick the files that are needed and put it into a
separate output, but I currently don't have time for this so I'm marking
this with XXX to get back to it later.

Signed-off-by: aszlig <aszlig@nix.build>