Fix checking of NAR hashes

*headdesk*
*headdesk*
*headdesk*

So since commit 22144afa8d, Nix hasn't
actually checked whether the content of a downloaded NAR matches the
hash specified in the manifest / NAR info file.  Urghhh...

src/libstore/build.cc

@@ -2749,6 +2749,7 @@ void SubstitutionGoal::finished()
     logPipe.readSide.close();
 
     /* Get the hash info from stdout. */
+    string dummy = readLine(outPipe.readSide);
     string expectedHashStr = statusOk(status) ? readLine(outPipe.readSide) : "";
     outPipe.readSide.close();