Publishing docs to GitHub pages fails because set-env has been deprecated

[jeffngo]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Build was run locally and any changes were pushed
• Lint has passed locally and any fixes were made for failures

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

The Publishing docs to GitHub pages GitHub action has been failing for a couple months with this error message:
The set-env command is disabled. Please upgrade to using Environment Files or opt into unsecure command execution by setting the ACTIONS_ALLOW_UNSECURE_COMMANDS environment variable to true. For more information see: https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/

What is the new behavior?

Set the CURRENT_BRANCH and LATEST_DOCS_BRANCH environment variables without using set-env as recommended by GitHub.

Does this introduce a breaking change?

• Yes
• No

Other information

I was able to test by pushing the changes to my VSDK fork and creating a branch from that fork.
Clean test run here: https://github.com/jeffngo/virtualization-sdk/actions/runs/424167649

[ankursarin]

Thanks for fixing this!