fix(build): Don't use Cachix as the binary cache during builds

Permission changes in the Travis CI Nix builders have caused this to
start failing, as the build user now has insufficient permissions to
use caches.

There may be a way to change the permissions instead, but in the
meantime we will just cause things to rebuild.

.travis.yml

@@ -12,7 +12,6 @@ before_script:
   - echo ${GOOGLE_KEY} | base64 -d > test-files/key.json
   - echo ${GCS_SIGNING_PEM} | base64 -d > test-files/gcs.pem
   - nix-env -f '<nixpkgs>' -iA cachix -A go
-  - cachix use nixery
 script:
   - test -z $(gofmt -l server/ build-image/)
   - nix-build --arg maxLayers 1 | cachix push nixery