Set bundle config rather than deprecated --system flag (#50497)

.cirrus.yml

@@ -456,7 +456,8 @@ task:
     - date
     - which flutter
     - bundle --version
-    - sudo bundle install --system --gemfile=dev/ci/mac/Gemfile
+    - bundle config set system 'true'
+    - sudo bundle install --gemfile=dev/ci/mac/Gemfile
     - git clean -xffd --exclude=bin/cache/
     - git fetch origin
     - git fetch origin master # To set FETCH_HEAD, so that "git merge-base" works.

dev/ci/docker_linux/Dockerfile

@@ -123,7 +123,8 @@ RUN gem install bundler -N
 COPY ci/docker_linux/Gemfile /Gemfile
 COPY ci/docker_linux/Gemfile.lock /Gemfile.lock
 
-RUN bundle install --system
+RUN bundle config set system 'true' && \
+  bundle install --system
 
 # Install goldctl, for Golden testing
 # Last updated 2020-02-04 (update to rebuild Dockerfile with latest goldctl)