internal/pkg: expose underlying reader

This will be fully implemented in httpArtifact in a future commit.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/rosa/busybox.go

@@ -16,7 +16,7 @@ import (
 // busyboxBin is a busybox binary distribution installed under bin/busybox.
 type busyboxBin struct {
 	// Underlying busybox binary.
-	bin pkg.File
+	bin pkg.FileArtifact
 }
 
 // Kind returns the hardcoded [pkg.Kind] value.