Make the build shell pure to avoid accidental host environment deps

Previously we got nix from the host env without even realizing it.  As soon as
it upgraded things broke.

.gitlab-ci.yml

@@ -69,7 +69,7 @@ unit-tests:
     - |
       set -x
       # GRID is set in one of the "instantiations" of this job template.
-      nix-shell --run "morph build --show-trace morph/grid/${GRID}/grid.nix"
+      nix-shell --pure --run "morph build --show-trace morph/grid/${GRID}/grid.nix"
 
 
 morph-build-localdev: