chore(build): Use separate GCS bucket for CI runs

This has become an issue recently with changes such as GZIP
compression, where CI runs no longer work because they conflict with
the production bucket for the public instance.

tools/nixery/.travis.yml

@@ -26,7 +26,7 @@ script:
     docker run -d -p 8080:8080 --name nixery \
       -v ${PWD}/test-files:/var/nixery \
       -e PORT=8080 \
-      -e BUCKET=nixery-layers \
+      -e BUCKET=nixery-ci-tests \
       -e GOOGLE_CLOUD_PROJECT=nixery \
       -e GOOGLE_APPLICATION_CREDENTIALS=/var/nixery/key.json \
       -e GCS_SIGNING_ACCOUNT="${GCS_SIGNING_ACCOUNT}" \