Switch from set-output to github environment files

[roc]

Hello!

Github has changed the way outputs get passed around to now use environment files.

Full details are in this blog post on the topic: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

This is a very tiny change that should quieten deprecation warnings in the workflow runner, and hopefully ensure this action continues to work when Github finally gets around to removing set-output.

p.s. Thanks for writing the action, it's dead helpful 💖

[koslib]

Looks good @roc and thanks for contributing! I hadn't noticed this change to be honest, and really appreciate your input! Love that you find this action useful as well :D

[koslib]

@roc I had sadly to revert this PR as it caused problems (see more at #48). Presumably it has something to do with the multi-line responses, I tried to find a solution to that but failed during my own attempts.

Had you tested this change out successfully before submitting? Is there any chance you can submit it with support for multi-line responses too?

[roc]

Ah I'm so sorry, that did work for us, but we're using v1.25.2.

I've tested with multiline outputs in this branch: https://github.com/koslib/helm-eks-action/compare/master...roc:helm-eks-action:github-output?expand=1

A successful run using that action is here: https://github.com/nhsx/standards-registry/actions/runs/3629725119/jobs/6122265491#step:7:72

Though I've noticed there's still an output warning! 🙄

Checking very briefly it seems like these are the only other points where that could be an issue in the action, so it may well be us causing the error at this point https://github.com/koslib/helm-eks-action/blob/master/.github/workflows/dockerhub.yaml#L15-L19