Update charmcraft to build bionic, run bionic/focal

This updates the charmcraft.yaml to correctly build on bionic and also
run on focal so that 1.6 can be used on bionic.

Change-Id: Id826ed806d848d03d59cd32d550a5f70d0970691

charmcraft.yaml

@@ -10,7 +10,8 @@ parts:
       - libpq-dev  # for the `pg_config` executable
     override-build: |
       apt-get install ca-certificates -y
-      tox -e build-reactive
+      pip3 install --user "tox==3.18"
+      ~/.local/bin/tox -e build-reactive
     override-stage: |
       echo "Copying charm to staging area: $CHARMCRAFT_STAGE"
       NAME=$(ls $CHARMCRAFT_PART_BUILD/build/builds)
@@ -23,14 +24,13 @@ parts:
 bases:
   - build-on:
       - name: ubuntu
-        channel: "20.04"
+        channel: "18.04"
         architectures:
           - amd64
-          - s390x
-          - ppc64el
-          - arm64
     run-on:
       - name: ubuntu
-        channel: "20.04"
+        channel: "18.04"
+        architectures: [amd64, s390x, ppc64el, arm64]
       - name: ubuntu
-        channel: "21.10"
+        channel: "20.04"
+        architectures: [amd64, s390x, ppc64el, arm64]