Github Actions: Make java spotless step optional

Add `continue-on-error: true` so it does not block cross-tests from
running.
diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index d963b0a..0b35d30 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -169,6 +169,7 @@
         run: |
           cd lib/java
           gradle spotlessCheck
+        continue-on-error: true
 
       - name: Run ktfmtCheck for Kotlin
         run: |