[cfe] Pass --no-sound-null-safety explicitly
[cfe] Pass --no-sound-null-safety explicitly

This prepares for changing sound null safety default.

Closes https://github.com/dart-lang/sdk/issues/50102

Change-Id: I88aa7994dd5542895c829d7777dd42d3b9438a29
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/262321
Commit-Queue: Johnni Winther <0f64ef784d57ffb064b634040f9eeba0d33d0be4@google.com>
Reviewed-by: Chloe Stefantsova <0b401402caf5616842b565557744ed97c1e5b2a2@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk