Update GitHub Actions workflows to use Env files

[Leandro Ostera]

We're getting a bunch of warnings about set-env being deprecated. e.g: https://github.com/AbstractMachinesLab/caramel/actions/runs/339535013

The set-env command is deprecated and will be disabled soon. We should upgrade to using Environment Files.

For whoever picks this up, here's a guide: https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/