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

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.