Set all envs with a given prefix

[JonZeolla]

I use task to do OpenTofu deployments, and often need to set a number of env vars with a TF_VAR_ prefix. I also include reusable taskfiles for standard actions.

Since the caller of these reusable actions may set arbitrary TF_VAR_ variables, I'd like a dynamic way to pass all of these more concisely by specifying the prefix.