Pin .NET 11 Preview 7 SDK in PR CI

[davidortinau]

Description

Pin pull request CI to .NET SDK 11.0.100-preview.7.26381.103 instead of the floating 11.0.x channel. This aligns workload installation with the Preview 7 baseline merged in #775.

The floating channel resolved SDK 11.0.100-preview.6.26359.118 and Preview 6 Android/MAUI workloads. Builds against the final Preview 7 MAUI packages then failed with CS1705 because Microsoft.Maui uses Java.Interop 99.61 while the installed workload references version 10.

Scope

• Keep 10.0.x unchanged.
• Change only the .NET 11 SDK selector in build-pr.yml.
• Do not change sample code or other workflow logic.

Validation

• Parsed build-pr.yml as YAML.
• Confirmed the final PR diff is one file with one insertion and one deletion.
• Confirmed setup-dotnet v4 supports exact prerelease SDK versions.
• Confirmed the official dotnet-install dry run resolves public payload URLs for the exact Preview 7 SDK.
• Completed pre-push review with no merge-blocking findings.

[davidortinau]

@jfversluis Release-day CI unblock. This one-line PR-build SDK pin has passed both macOS and Windows checks. It fixes the confirmed Preview 6 workload / Preview 7 package CS1705 mismatch so #776 and #777 can rerun. Human approval is the only remaining blocker.