cross-tests: set-output deprecation warning #47

The action results in the following warning (e.g. see here):

Run RustCrypto/actions/cross-tests@master
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/

I don't understand why we get this warning. set-output is not explicitly by the job and I couldn't find its invocation in logs.