refactor(kernel): report kernel download progress through the sink

get_default_kernel emits Progress events for the APK transfer instead
of drawing a bar to stderr from inside the library.

crates/ecr-cli/src/main.rs

@@ -63,7 +63,7 @@ fn main() -> Result<()> {
             }
             None => {
                 veprintln!("QEMU mode: downloading default kernel...");
-                kernel::get_default_kernel(cache.dir(), &arch)?
+                kernel::get_default_kernel(cache.dir(), &arch, &ecr::NoProgress)?
             }
         };