Accept non-process arguments in functions for easier testing

[jghiloni]

Describe the enhancement
Maybe I missed something completely, but it'd be great for functions like core.getInput or core.setOutput to take a NodeJS.ProcessEnv object as an optional argument, defaulting to process.env, to make testing JS actions easier and cleaner.

Code Snippet
If your action code is something like

import * as core from "@actions/core";

const myInput = core.getInput("my_input_var"); 

it could be something like

import * as core from "@actions/core";

const myInput = core.getInput("my_input_var", undefined /* options */, {
  INPUT_MY_INPUT_VAR: "some-value",
});

Additional information
Again, if that's already possible, sorry for the extra toil!