Fix FileStream leak in ClassPath.IsJmodFile (#1424)

Add using to File.OpenRead in IsJmodFile so the stream is disposed on all return paths, matching the pattern used by the companion IsJarFile method.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jonathanpeppers

src/Xamarin.Android.Tools.Bytecode/ClassPath.cs

@@ -145,7 +145,7 @@ public static bool IsJmodFile (string jmodFile)
 			if (jmodFile == null)
 				throw new ArgumentNullException (nameof (jmodFile));
 			try {
-				var f = File.OpenRead (jmodFile);
+				using var f = File.OpenRead (jmodFile);
 				var h = new byte[4];
 				if (f.Read (h, 0, h.Length) != 4) {
 					return false;