Replace use of deprecated "set-output" in CI GitHub Actions.

[tcal-x]

The following warning is given with most of the workflows in this repo's CI:

The `set-output` command is deprecated and will be disabled soon. 
Please upgrade to using Environment Files. 
For more information see: 
https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

test-projects.yml does use set-output directly.

However the warning also occurs in workflows that don't use set-output directly, such as install-use-oxide-conda from oxide.yml. It seems that actions/setup-python@v3 is directly or indirectly using set-output, so this will need fixing upstream.