Try fixing proguard config for nightly builds

Author: jmartinesp

libraries/pushproviders/firebase/build.gradle.kts

@@ -51,6 +51,7 @@ android {
         register("nightly") {
             isMinifyEnabled = true
             consumerProguardFiles("consumer-proguard-rules.pro")
+            proguardFiles("proguard-rules.pro")
             matchingFallbacks += listOf("release")
             resValue(
                 type = "string",

libraries/pushproviders/firebase/proguard-rules.pro

@@ -0,0 +1,5 @@
+# Used for unit tests
+
+-dontwarn java.lang.invoke.StringConcatFactory
+
+-keep class io.element.android.libraries.pushproviders.firebase.** { *; }