demo: Make improvements/corrections to demo test

- The "last release" tests weren't using "latest-stable", which is
  *actually* the last release.
- The grafana-against-HEAD test wasn't actually using HEAD because the
  env var definitions in the step weren't the right syntax.
- The step dependency graph has been adjusted so now the demo pipeline
  runs in about 7 min instead of 20 min.

Change-Id: I48e8f6e120e29871875b38954f21b754b8fdb4a9
Reviewed-on: https://gerrit.readyset.name/c/readyset/+/8986
Tested-by: Buildkite CI
Reviewed-by: Mohamed Yasser Abdelhamed Abdeen <[email protected]>

Author: ronh-rs

.buildkite/test-demo.sh

@@ -23,7 +23,7 @@ fi
 # Allows setting this manually if testing locally
 if [ -z "$READYSET_DOCKER_IMG" ]; then
   # Use the latest image that hasn't been pushed to docker hub yet (if any) to test it before releasing it
-  READYSET_DOCKER_IMG="public.ecr.aws/z3o1l5n4/readyset:latest"
+  READYSET_DOCKER_IMG="public.ecr.aws/z3o1l5n4/readyset:latest-stable"
 fi
 
 sed_in_place() {