Fix(pynn-brainscales): Enforce correct googletest version

This is a hack to enforce a reinstallation of meta-brainscales.
(The underlying problem is that pynn-brainscales does not specify all
the dependencies but only via meta-brainscales; and there's already one
installed and spack does not easily install another one ;).)

spack.yaml

@@ -26,4 +26,4 @@ spack:
     - py-formencode %gcc@10.3.0
     - tvb-framework ^binutils+ld+gold %gcc@10.3.0
     - meta-brainscales %gcc@10.3.0
-    - pynn-brainscales@1.0-rc2 %gcc@10.3.0
+    - pynn-brainscales@1.0-rc2 ^googletest@1.11.0:+gmock %gcc@10.3.0