[infra] Add cfe NNBD builder to the test matrix
[infra] Add cfe NNBD builder to the test matrix

See also https://dart-review.googlesource.com/c/sdk/+/130860/

Change-Id: Ic46a8bb13c2a2f73f1fc03c1520693df8ece39bf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/130861
Reviewed-by: Johnni Winther <0f64ef784d57ffb064b634040f9eeba0d33d0be4@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