Symlink python3 to python for gerrit image build

Gerrit's bazel rules are looking for python which doesn't exist on our
images. Add a python symlink to python3 until
https://gerrit-review.googlesource.com/c/gerrit/+/298903 is in a release,
which seems likely to be 3.5.

Change-Id: I1c15cceac1c9bbf435ed23bed7c1e3fe868f05ff

playbooks/zuul/gerrit/repos.yaml

@@ -15,6 +15,16 @@
           - unzip
         state: present
 
+    # Until Gerrit 3.5 we need to work around scripts looking for bare
+    # python instead of python3.
+    # https://gerrit-review.googlesource.com/c/gerrit/+/298903
+    - name: Install python symlink
+      become: true
+      ansible.builtin.file:
+        src: /usr/bin/python3
+        dest: /usr/bin/python
+        state: link
+
     - name: Move common plugin repos into gerrit tree
       command: "mv -f {{ item }} ../gerrit/plugins"
       args: