You could certainly do it. No point having multiple EBS volumes lying around - just create a snapshot of the volume with the git checkout / build cache after each new build is done.

When you want to build again, create the instance and then recreate the EBS volume from the snapshot and attach it to the new instance. Pull the latest set of changes from the git repo and build with the old cache!

Obviously there are cache purging considerations (e.g. starting from scratch once per week/month) you could optimise as well.