github ci: Add missing coverage command

The way to recover coverage was changed in antsibull recently:
ansible-community/antsibull-build@c1cda56

Author: dmsimard

.github/workflows/antsibull-build-6.yml

@@ -40,6 +40,7 @@ jobs:
         working-directory: antsibull
         run: |
           ansible-playbook -vv playbooks/build-single-release.yaml \
+            -e 'antsibull_build_command="poetry run coverage run -p --source antsibull -m antsibull.cli.antsibull_build"' \
             -e antsibull_build_file=ansible-6.build \
             -e antsibull_ansible_version=6.0.0a1 \
             -e antsibull_data_dir="{{ antsibull_data_git_dir }}/6" \

.github/workflows/antsibull-build-default.yml

@@ -40,7 +40,9 @@ jobs:
       - name: Test building a release with the defaults
         working-directory: antsibull
         run: |
-          ansible-playbook -vv playbooks/build-single-release.yaml -e antsibull_data_reset=false
+          ansible-playbook -vv playbooks/build-single-release.yaml \
+          -e 'antsibull_build_command="poetry run coverage run -p --source antsibull -m antsibull.cli.antsibull_build"' \
+          -e antsibull_data_reset=false
 
       - name: Combine and upload coverage stats
         working-directory: antsibull