that didn't work; try a newer image

.circleci/config.yml

@@ -148,7 +148,11 @@ jobs:
       # one day someone pushed a bad revision to it and our CI broke.  So now
       # we just pin some recent version.  Who would have thought a floating
       # dependency would cause build instability?
-      - image: "nixos/nix:2.5.1"
+      #
+      # This pre-release image in particular fixes a problem in the 2.5.1
+      # image where no CA certificates are available.
+      # https://github.com/NixOS/nix/issues/5797
+      - image: "nixos/nix:2.6.0pre20211228_f7d22f4-"
 
     # Tahoe-LAFS requires more memory than we get from the default resource
     # class and sometimes we have to build it.
@@ -167,18 +171,6 @@ jobs:
       NIXPKGS: "https://github.com/NixOS/nixpkgs/archive/28abc4e43a24d28729509e2d83f5c4f3b3418189.tar.gz"
 
     steps:
-      - run:
-          # Work around a bug in the 2.5.1 Docker image that prevents it from
-          # having any CA certificates to use to validate any certificates it
-          # encounters (and thus makes it incapable of talking to our binary
-          # caches).
-          #
-          # The work-around is from a comment on the issue
-          # https://github.com/NixOS/nix/issues/5797
-          name: "Fix CA Certificates"
-          command: |
-            ln -s $NIX_SSL_CERT_FILE /etc/ssl/certs/
-
       - run:
           name: "Set up Cachix"
           command: |