CI: Disable JDK auto-download in Tangled build
The foojay resolver tries to download a JetBrains JDK 21 which
fails to unpack in CI. Disable auto-download so Gradle uses the
Nix-provided jdk21_headless instead.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>