gitlab: Use $CI_DEFAULT_BRANCH instead of hardcoded 'master'

We want to skip the checkpatch and DCO signoff jobs when
pushing to the default branch. Currently this branch is
called 'master', but we don't need to hardcode this in
the CI configuration, because the $CI_DEFAULT_BRANCH
env variable exposes it.

References:

- https://sfconservancy.org/news/2020/jun/23/gitbranchname/
- https://about.gitlab.com/blog/2021/03/10/new-git-default-branch-name/

Suggested-by: Savitoj Singh <savsingh@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210525153826.4174157-1-philmd@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>

.gitlab-ci.d/static_checks.yml

@@ -6,7 +6,7 @@ check-patch:
   script: .gitlab-ci.d/check-patch.py
   except:
     variables:
-      - $CI_PROJECT_NAMESPACE == 'qemu-project' && $CI_COMMIT_BRANCH == 'master'
+      - $CI_PROJECT_NAMESPACE == 'qemu-project' && $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH
   variables:
     GIT_DEPTH: 1000
   allow_failure: true
@@ -19,6 +19,6 @@ check-dco:
   script: .gitlab-ci.d/check-dco.py
   except:
     variables:
-      - $CI_PROJECT_NAMESPACE == 'qemu-project' && $CI_COMMIT_BRANCH == 'master'
+      - $CI_PROJECT_NAMESPACE == 'qemu-project' && $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH
   variables:
     GIT_DEPTH: 1000