Install v2.20.0 of docker-compose-plugin (#6997)

Work around the bug[1] in docker-compose-plugin v2.19.1, which is
included in the current github actions ubuntu 20.04 runner image[2].

[1] docker/compose#10777
[2] actions/runner-images#7916

Part of #6998

Author: aarongable

.github/workflows/boulder-ci.yml

@@ -73,6 +73,11 @@ jobs:
       - uses: actions/checkout@v3
         with:
           persist-credentials: false
+      
+      # TODO(#6998): Remove this step when the ubuntu-20.04 image has v2.20.0+.
+      # Install instructions copied from https://docs.docker.com/compose/install/linux/#install-the-plugin-manually
+      - name: Update docker compose plugin
+        run: mkdir -p ${DOCKER_CONFIG:-$HOME/.docker}/cli-plugins && curl -SL https://github.com/docker/compose/releases/download/v2.20.0/docker-compose-linux-x86_64 -o ${DOCKER_CONFIG:-$HOME/.docker}/cli-plugins/docker-compose && chmod +x ${DOCKER_CONFIG:-$HOME/.docker}/cli-plugins/docker-compose
 
       - name: Docker Login
         # You may pin to the exact commit or the version.