Drop CircleCI caching of /nix/store

At some point some issue was introduced into the nix store cache on CircleCI.
Restoring the cache is currently half failing and causes `nix-build` to fail
to find `nixpkgs`.

.circleci/config.yml

@@ -19,16 +19,6 @@ jobs:
     docker:
       - image: "nixos/nix:2.2.1"
     steps:
-      - run:
-          name: "Install CA Certificates"
-          command: |
-            # Required for cache and artifact interactions.  Though we use a
-            # nix image, it's actually an alpine base...  The CircleCI cache
-            # management and artifact uploader doesn't know how to use the nix
-            # ca bundle we could install.
-            apk update
-            apk add ca-certificates
-
       - run:
           name: "Install Git"
           command: |
@@ -51,8 +41,8 @@ jobs:
           command: |
             # Required for cache and artifact interactions.  Though we use a
             # nix image, it's actually an alpine base...  The CircleCI cache
-            # management and artifact uploader doesn't know how to use the nix
-            # ca bundle we could install.
+            # management (which we stopped using) and artifact uploader don't
+            # know how to use the nix ca bundle we could install.
             apk update
             apk add ca-certificates
 
@@ -62,10 +52,6 @@ jobs:
             # Required for the checkout step
             nix-env -i git openssh
 
-      - restore_cache:
-          keys:
-            - "v1-nix-store"
-
       - "checkout"
 
       - run:
@@ -73,11 +59,6 @@ jobs:
           command: |
             nix-build
 
-      - save_cache:
-          key: "v1-nix-store"
-          paths:
-            - "/nix/store"
-
       - store_artifacts:
           path: "result/docs"
           destination: "docs"