[actions] exclude Xcode 26 beta 6

[glessard]

Xcode 26b6's macOS SDK doesn't support backdeployment of Span and friends, guaranteeing compilation failures in the modules that use them.
This is fixed in Xcode 26 17A321, released on September 9th.

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• I've updated the documentation if necessary.