Fix application distribution for nightly build.

app/build.gradle.kts

@@ -125,6 +125,10 @@ android {
 
             firebaseAppDistribution {
                 artifactType = "APK"
+                // We upload the universal APK to fix this error:
+                // "App Distribution found more than 1 output file for this variant.
+                // Please contact firebase-support@google.com for help using APK splits with App Distribution."
+                artifactPath = "$rootDir/app/build/outputs/apk/nightly/app-universal-nightly.apk"
                 // This file will be generated by the GitHub action
                 releaseNotesFile = "CHANGES_NIGHTLY.md"
                 groups = "external-testers"