CI: force IPv4 for Gradle so Maven Central is reachable

The runner host has no working IPv6 route; Maven Central (AAAA records) was
'Network is unreachable' while IPv4-only CDNs worked. Set
JAVA_TOOL_OPTIONS=-Djava.net.preferIPv4Stack=true so all build JVMs use IPv4.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.forgejo/workflows/build-apk.yml

@@ -14,6 +14,10 @@ jobs:
     env:
       ANDROID_HOME: /opt/android-sdk
       ANDROID_SDK_ROOT: /opt/android-sdk
+      # Force IPv4 for all JVMs (Gradle launcher, daemon, Kotlin/CMake workers).
+      # The runner host has no working IPv6 route, so Maven Central (which has
+      # AAAA records) was unreachable — this makes Java ignore AAAA and use IPv4.
+      JAVA_TOOL_OPTIONS: -Djava.net.preferIPv4Stack=true
       GRADLE_OPTS: -Dorg.gradle.jvmargs=-Xmx4g -Dorg.gradle.daemon=false
     steps:
       - name: Checkout