Add /var/cache/dhall for bubblewrap

- add /var/cache/dhall in trusted_rw_paths to allow cache update
during dhall-diff in trusted repos
- add /var/cache/dhall in untrusted_ro_paths for untrusted repos

This will allow the use of dhall tooling on the executors

Change-Id: I34e79eddaee9af65435a469f243cf39a8c5e459d

controllers/static/zuul/zuul.conf

@@ -19,7 +19,8 @@ log_config=/var/lib/zuul/zuul-web-logging.yaml
 private_key_file=/var/lib/zuul-ssh/..data/priv
 # Remove /etc/pki when https://review.opendev.org/c/zuul/zuul/+/884781 is released
 trusted_ro_paths=/etc/pki
-untrusted_ro_paths=/etc/pki
+trusted_rw_paths=/var/cache/dhall
+untrusted_ro_paths=/etc/pki:/var/cache/dhall
 log_config=/var/lib/zuul/zuul-executor-logging.yaml
 manage_ansible=false
 
@@ -37,4 +38,3 @@ client_id=zuul-client
 
 [statsd]
 server=localhost
-