.github/workflows/ci.yml: skip Ruby in Docker jobs

In the "build-docker" and "build-docker-cross" jobs in our CI GitHub
Actions workflow we currently install Ruby using the ruby/setup-ruby
action, which has been true since we introduced the first of these
jobs in commit 212c051 of PR git-lfs#3932.

However, unlike the corresponding jobs in our release workflow, where
we need the Ruby interpreter in order to run our script/packagecloud.rb
utility at the end of the job, we do not actually require Ruby for the
CI versions of these jobs, so we can simply remove the ruby/setup-ruby
step from them now.

Author: chrisd8088

.github/workflows/ci.yml

@@ -179,7 +179,6 @@ jobs:
         fetch-depth: 0
         persist-credentials: false
         ref: ${{ github.ref }}
-    - uses: ruby/setup-ruby@v1
     - run: git clone https://github.com/git-lfs/build-dockers.git "$HOME/build-dockers"
     - run: (cd "$HOME/build-dockers" && ./build_dockers.bsh)
     - run: ./docker/run_dockers.bsh --prune
@@ -196,7 +195,6 @@ jobs:
         fetch-depth: 0
         persist-credentials: false
         ref: ${{ github.ref }}
-    - uses: ruby/setup-ruby@v1
     - run: |
         echo '{"experimental": true}' | sudo tee /etc/docker/daemon.json
         sudo systemctl restart docker.service