tart pull: open the VM directory after pulling under a lock (#936)

Sources/tart/Commands/Pull.swift

@@ -47,8 +47,5 @@ struct Pull: AsyncParsableCommand {
     defaultLogger.appendNewLine("pulling \(remoteName)...")
 
     try await VMStorageOCI().pull(remoteName, registry: registry, concurrency: concurrency, deduplicate: deduplicate)
-
-    // to explicitly set the image as being accessed so it won't get pruned immediately
-    _ = try VMStorageOCI().open(remoteName)
   }
 }

Sources/tart/VMStorageOCI.swift

@@ -239,6 +239,9 @@ class VMStorageOCI: PrunableStorage {
       // are excluded from garbage collection
       VMDirectory(baseURL: vmURL(name)).markExplicitlyPulled()
     }
+
+    // to explicitly set the image as being accessed so it won't get pruned immediately
+    _ = try VMStorageOCI().open(name)
   }
 
   func linked(from: RemoteName, to: RemoteName) -> Bool {