Fixes skiptools#675
The issue occurred because the `skip` project is built without `NonisolatedNonsendingByDefault`, which means that `runGradleTests` and `invokeGradle` were `@concurrent` by legacy default, forbidden to access `self` or any `task-isolated` members.
The generated `XCSkipTests` file in the end-user's project, however, _is_ `NonisolatedNonsendingByDefault` on Swift 6.2, which means that `testSkipModule` method became task-isolated. (I'm a little unclear on how/why the test became task-isolated, but it did.)
We could have fixed this by updating `skip` to Swift 6.2+ and enabling `NonisolatedNonsendingByDefault` and other "Approachable Concurrency" features there, but this is easier, and will continue to work if/when we upgrade `skip` in the future. (When `skip` does decide to enable `NonisolatedNonsendingByDefault`, these annotations will simply reiterate the default.)