Quick-fix to add nest's lib64 site-packages to PYTHONPATH

(cherry picked from commit 121d8497)

packages/nest/package.py

@@ -176,3 +176,8 @@ class Nest(CMakePackage):
     @on_package_attributes(run_tests=True)
     def installcheck(self):
         make("installcheck")
+
+    # quick-fix to add lib64/python3.8/site-packages to the PYTHONPATH;
+    # this should happen automatically for packages extending Python...
+    def setup_run_environment(self, env):
+        env.prepend_path('PYTHONPATH', self.prefix.lib64.join('python3.8/site-packages'))