Ignore flaky test on Linux, too

The GitHub Actions runners are apparently slow to deliver filesystem
notifications, which is exactly the scenario that makes this test
useless. It's already disabled on Windows; there's no reason for it to
cause the CI build to fail all the time when it's the only failing test.

src/LibChorusTests/notes/AnnotationRepositoryTests.cs

@@ -224,7 +224,7 @@ public void AddAnnotation_NotifiesIndices()
 		// the setup of the watcher or the code in Save that tries to prevent the notifications at all,
 		// which is more than half the code this test wants to exercise.
 		[Test]
-		[Platform(Exclude = "Win", Reason = "flaky (on both platforms)")]
+		[Ignore("flaky (on both platforms)")]
 		public void ExternalFileModification_NotifiesIndices_ButSaveDoesNot()
 		{
 			const int SleepTime = 25; // milliseconds