Add hxtorch@2.0-rc8 to environment

This reverts commit f64d31a2.

spack.yaml

@@ -30,7 +30,7 @@ spack:
     - py-formencode
     - tvb-framework ^binutils+ld+gold
     - pynn-brainscales@2.0-rc8 ^log4cxx@0.10.0 ^googletest@1.11.0:+gmock
-    #- hxtorch@2.0-rc6 ^log4cxx@0.10.0 ^googletest@1.11.0:+gmock
+    - hxtorch@2.0-rc8 ^log4cxx@0.10.0 ^googletest@1.11.0:+gmock
     - py-neo
     #- py-cerebstats
     #- py-cerebunit