Write npmrc file in CI only

locally yarn would fail because it can't expand the env var because it
is usually missing:

error An unexpected error occurred: "Failed to replace env in config: ${NPM_TOKEN}"

.gitlab-ci.yml

@@ -23,6 +23,7 @@ publish:
       - publish
       - master
   script:
+    - echo '//registry.npmjs.org/:_authToken=${NPM_TOKEN}' > .npmrc
     - sudo npm install --global can-npm-publish
     - yarn config set registry https://registry.npmjs.org/
     - yarn config set cache-folder "$CI_PROJECT_DIR/_yarn_cache"

.npmrc

-//registry.npmjs.org/:_authToken=${NPM_TOKEN}
\ No newline at end of file