CI: Replace the deprecated `::set-output` command by writing to $GITHUB_OUTPUT in...

CI: Replace the deprecated `::set-output` command by writing to $GITHUB_OUTPUT in matrix.py

As announced in

https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

the `::set-output` command is deprecated, because processes during the workflow
execution might output untrusted information that might include the
`::set-output` command, thus allowing these untrusted information to hijack the
build.

The replacement is writing to the file indicated by the `$GITHUB_OUTPUT`
environment variable.