Function to get the path for the env file, if one is used

I think it would be useful to have a function that would return the path of the current dotenv file, if one is currently being used. My use case is to pass the currently loaded dotenv file to `docker run --env-file `. This seems nicer than specifying the dotenv path as an argument to the target, because it will load the expected dotenv file by default, and allow you to override it with `just --dotenv-filename` or `just --dotenv-path`, and the use of docker would be fully abstracted from the `just` user.

Start by tracing how --dotenv-filename and --dotenv-path select and load the current dotenv file. Determine the function's behavior when no dotenv file is used, then add coverage for the returned path and run the relevant test suite.

Written by the indexing model from the issue text.