hydra: Switch back to "-" instead of "@" in package job names.

Fixes a regression introduced in ede121de42.
Reported by Mark H Weaver <mhw@netris.org>.

* build-aux/hydra/gnu-system.scm (job-name): Pass "-" as the second
argument to 'package-full-name'.

build-aux/hydra/gnu-system.scm

@@ -264,7 +264,8 @@ all its dependencies, and ready to be installed on non-GuixSD distributions.")
 
 (define job-name
   ;; Return the name of a package's job.
-  (compose string->symbol package-full-name))
+  (compose string->symbol
+           (cut package-full-name <> "-")))
 
 (define package->job
   (let ((base-packages