A couple of things to try in addition to or before trying a local build…
One simple idea to try would be to include pip as a dependency. For several months I had been seeing it in the MyBinder build stream noted that something was saying soon including pip as a dependency is required. So I started including it a while ago in new or updated environment.yml. I note that yours doesn’t have it.
See an example here:
You have a complex set of requirements in your environment.yml and then you specify an old version of Python. Sometimes mixing pinning and unpinning is a bad idea. So a related idea would be to listing pip earlier and also unpinning everything in a simpler, separate repo and then build back up to see where breaks.