Update our gitea test to handle new git behavior

Git was recently updated to fix a security issue that prevents git from
operating on a repo as userA if the repo is owned by userB. In our gitea
tests we use our local zuul repo clone of system-config to push back
into gitea to get some real content into gitea. We were operating as
root but zuul owns that repo. Update the commands to run as zuul to
workaround the git error.

Change-Id: I87105bae4bdd69465cce4d5bc412241dc1c88623

playbooks/test-gitea.yaml

@@ -22,8 +22,12 @@
       when: '"Zuuls new description" not in zuul_content.content'
     - name: Configure ssh for pushing into test gitea
       shell: |
-        ssh-keyscan -t rsa -p 222 localhost >> .ssh/known_hosts
+        ssh-keyscan -t rsa -p 222 localhost >> /home/zuul/.ssh/known_hosts
         ssh-keygen -t ed25519 -f /home/zuul/.ssh/id_ed25519 -C giteagerrittest -P ''
+      # We become zuul here because we need to git push has zuul later to
+      # ensure the owner of the git repo is the one operating on it.
+      become: true
+      become_user: zuul
     - name: Register new gitea gerrit pubkey value
       slurp:
         src: /home/zuul/.ssh/id_ed25519.pub
@@ -48,6 +52,11 @@
         cd /home/zuul/src/opendev.org/opendev/system-config
         git remote add test-gitea ssh://git@localhost:222/opendev/system-config
         GIT_SSH_COMMAND='ssh -i /home/zuul/.ssh/id_ed25519' git push -f test-gitea master
+      # We run this as zuul because zuul owns the git repo in /home/zuul
+      # and git by default does not want to operate on a git repo as another
+      # user.
+      become: true
+      become_user: zuul
     - name: Clone system-config to check that partial clones aren't breaking stuff
       shell: |
         cd /tmp