Put nix in the shell environment

We don't want to inherit nix from the host environment because who knows if it is compatible with our software or not.

.gitlab-ci.yml

@@ -67,8 +67,9 @@ unit-tests:
   stage: "build"
   script:
     - |
+      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:
@@ -111,7 +112,7 @@ system-tests:
   timeout: "3 hours"
   stage: "build"
   script:
-    - "nix-build --attr system-tests"
+    - "nix-shell --pure --run 'nix-build --attr system-tests'"
 
 # A template for a job that can update one of the grids.
 .update-grid: &UPDATE_GRID