sysext: do not attempt to unlock images interactively

These images are not using a passphrase, they are using keys
or at most TPM-based sealing (not yet implemented, for contexts).

Do not use the interactive helper, as it will block and ask the
user for a password if it fails to find the signing cert, which
is not useful for this tool.

Fixes https://github.com/systemd/systemd/issues/33179

src/sysext/sysext.c

@@ -1830,10 +1830,7 @@ static int merge_subprocess(
                         if (r < 0)
                                 return r;
 
-                        r = dissected_image_decrypt_interactively(
-                                        m, NULL,
-                                        &verity_settings,
-                                        flags);
+                        r = dissected_image_decrypt(m, /* passphrase= */ NULL, &verity_settings, flags);
                         if (r < 0)
                                 return r;