Skip matrix jobs more cleanly

[jakepetroules]

Now disabled jobs will just disappear entirely, rather than appear with confusing unexpanded placeholders.

[ahoppen]

While this nicely cleans up the status checks on the PR (which I love), the workflows are now failing with Matrix vector '...' does not contain any values, for example in https://github.com/swiftlang/swift-syntax/actions/runs/19259914267, which causes a PR run failed email to be sent to the PR author.

[FranzBusch]

I wasn't in time to comment on this PR but we have been down this path in the swift-nio workflows and this approach is not sufficient enough as indicated by @ahoppen failure. We have tried many things in swift-nio to work around this and the only solution so far was to split the matrix generation and running the matrix into two distinct jobs. I think it would still be the best to see if we can port the approach that NIO came up with to this repo in the near term. Until then I would recommend to revert this change.