Pin pynn-brainscales to new version

I forgot to pin the version to the newly introduced rc2… spack decided to keep the old one → enforce new version.

spack.yaml

@@ -24,4 +24,4 @@ spack:
     - py-formencode %gcc@10.3.0
     - tvb-framework ^binutils+ld+gold %gcc@10.3.0
     - meta-brainscales %gcc@10.3.0
-    - pynn-brainscales %gcc@10.3.0
+    - pynn-brainscales@1.0-rc2 %gcc@10.3.0