As per my understanding, it is not possible to have Environment Variables on Slash.
While writing the Lambdas, some times it is essential to call other services and we need some sort of credentials or keys for this.
We can write the keys or credentials within the lambdas but it is somewhat insecured as all the source code needs to be checked in the codebase (ex github) and we dont want the secured keys to be leaked.
Usual solution which almost all hosted services provide is to securely add env variables which are only accessible on the server.
Note that we do the same with our servers, Kubernetes cluster and similar.
So in My opinion, Slash should provide a way to add variables which are only accessible when executing the lambda resolver.