Set ENV and Add Path commands deprecated

[polothy]

See https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/

Probably need to write to these new files: https://docs.github.com/en/free-pro-team@latest/actions/reference/workflow-commands-for-github-actions#environment-files

Can probably port what was done in the @actions/core package:

Add path https://github.com/actions/toolkit/blob/4f7fb6513a355689f69f0849edeb369a4dc81729/packages/core/src/core.ts#L63-L75

Set env https://github.com/actions/toolkit/blob/4f7fb6513a355689f69f0849edeb369a4dc81729/packages/core/src/core.ts#L35-L53

[ashutoshgngwr]

@polothy @sethvargo I have a fix for this in ashutoshgngwr@fdb2783 but the commit builds on top of #10.

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