Clear build cache during CI

Appears there is an issue where the build cache grows unbounded until it hits Github's limit (docker/build-push-action#252, moby/buildkit#1896). Clear the cache after builds to prevent this

.github/workflows/ci.yaml

@@ -181,3 +181,11 @@ jobs:
             BUILD_REF=${{ github.sha }}
             BUILD_REPOSITORY=${{ github.repository }}
             BUILD_VERSION=edge
+      # This ugly bit is necessary, or our cache will grow forever...
+      # Well until we hit GitHub's limit of 5GB :)
+      # https://github.com/docker/build-push-action/issues/252
+      # https://github.com/moby/buildkit/issues/1896
+      - name: 🚚 Swap build cache
+        run: |
+          rm -rf /tmp/.docker-cache
+          mv /tmp/.docker-cache-new /tmp/.docker-cache