fix(sdk-core): backup keychain creation to use correct privateMaterial

TICKET: WP-6057
Replaces the use of userPrivateMaterial with backupPrivateMaterial when adding the backup keychain in EcdsaMPCv2Utils. This ensures the correct private material is used for backup keychain generation.

Author: sasikumar-bitgo

modules/sdk-core/src/bitgo/utils/tss/ecdsa/ecdsaMPCv2.ts

@@ -321,7 +321,7 @@ export class EcdsaMPCv2Utils extends BaseEcdsaUtils {
     );
     const backupKeychainPromise = this.addBackupKeychain(
       bitgoCommonKeychain,
-      userPrivateMaterial,
+      backupPrivateMaterial,
       backupReducedPrivateMaterial,
       params.passphrase,
       params.originalPasscodeEncryptionCode