ci: do a full checkout of master for semantic-release

.taskcluster.yml

@@ -51,6 +51,7 @@ tasks:
                 git config --global user.email "bugmon@mozilla.com";
                 git config --global user.name "semantic-release";
                 git fetch origin master --tags;
+                git checkout -b master;
                 tox -e release
             when:
               branch: master