chore: fix clirr failure

Author: olavloite

google-cloud-spanner/clirr-ignored-differences.xml

@@ -822,4 +822,10 @@
     <method>java.lang.Object runTransaction(com.google.cloud.spanner.connection.Connection$TransactionCallable)</method>
   </difference>
 
+  <!-- Added external host option -->
+  <difference>
+    <differenceType>7012</differenceType>
+    <className>com/google/cloud/spanner/SpannerOptions$SpannerEnvironment</className>
+    <method>com.google.auth.oauth2.GoogleCredentials getDefaultExternalHostCredentials()</method>
+  </difference>
 </differences>