jjb: ensure /log files are writable for deletion

Bazel output of the action that compiles `DumpPlatformClassPath` is
written in a non -writable directory. When we wipe the `log` directory
with `find log/ --delete` that results in:

/usr/bin/find: cannot delete ‘/workspace/log/execroot/eventsWikimedia
 /bazel-out/k8-fastbuild/bin/external/bazel_tools/tools/jdk
 /platformclasspath_classes/DumpPlatformClassPath.class’: Permission denied

Unconditionally create the `log` directory and ensure all files are
writables before deletion.

Update gerrit-events-wikimedia-bazel-docker

Bazel issue bazelbuild/bazel#15616

Bug: T320251
Change-Id: Ia6dafa3e9bacd81c03e9ec35644c8a2893368e03

jjb/job-templates.yaml

@@ -70,6 +70,18 @@
     builders:
       - docker-castor-load
       - docker-src-dir
+
+      # Ensure /log files are writable before deleting - T320251
+      - shell: |
+          mkdir -m 2777 -p "log"
+      - docker-run:
+          image: docker-registry.wikimedia.org/buster:latest
+          entrypoint: /bin/chmod
+          options: >-
+              --volume "$WORKSPACE/log":/log
+          args: >-
+              -R u+w /log
+
       - docker-log-dir
       - docker-ci-src-setup-simple
       - docker-run-with-log-cache-src: