Add build-on/run-on to support Ubuntu 20.04

To fix the build jsonschema is pinned to the latest version that doesn't
depend (indirectly) on Rust.

Closes-Bug: #2033673
Change-Id: Id6777eb23b3b4cc6b0fb239a61d64502a5d74451

charmcraft.yaml

@@ -23,10 +23,13 @@ parts:
 bases:
   - build-on:
       - name: ubuntu
-        channel: "22.04"
+        channel: "20.04"
         architectures:
           - amd64
     run-on:
+      - name: ubuntu
+        channel: "20.04"
+        architectures: [amd64, s390x, ppc64el, arm64]
       - name: ubuntu
         channel: "22.04"
         architectures: [amd64, s390x, ppc64el, arm64]

test-requirements.txt

@@ -28,3 +28,4 @@ git+https://opendev.org/openstack/tempest.git#egg=tempest
 
 pyopenssl<=22.0.0
 croniter            # needed for charm-rabbitmq-server unit tests
+jsonschema<4.18.0   # jsonschema 4.18.0 depends on Rust (via rpds-py)