Disable Xcode-beta builds until one is installed

[madrob]

Xcode-latest currently points to the 26.1 release so it gives us a false sense of version coverage, and should be ignored.

Motivation:

We do not need to build twice with the same environment.

Modifications:

Set default value for Xcode latest beta enabled to false

Result:

One fewer job generated for the macOS build matrix.

[madrob]

I'm not sure why the matrix still generated a job for Xcode-latest here. Is it explicitly using the version committed to the base branch instead of the version I have in the PR?

https://github.com/apple/swift-nio/actions/runs/19340051165/job/55325750250?pr=3446

[rnro]

I'm not sure why the matrix still generated a job for Xcode-latest here. Is it explicitly using the version committed to the base branch instead of the version I have in the PR?

https://github.com/apple/swift-nio/actions/runs/19340051165/job/55325750250?pr=3446

We reference branch main in all our workflows so you need to push a commit redirecting the workflows to use your branch if you want to test it then back it out before merging. In this case I don't think that's necessary.