hydra-queue-runner: Send build notifications

Since our notification plugins are written in Perl, sending
notification from C++ requires a small Perl helper named
‘hydra-notify’.

src/script/hydra-evaluator

@@ -1,4 +1,4 @@
-#! /var/run/current-system/sw/bin/perl
+#! /run/current-system/sw/bin/perl
 
 use strict;
 use utf8;