fix: re-pin Flutter's JDK path on devbox init

Flutter caches its JDK location in a per-user global config file
(~/.config/settings), not per-project. On a shared build host that
cross-contaminates between projects: whichever project last ran
without pinning inherits a stale jdk-dir pointing at another
project's (possibly already-gone) build directory, which breaks the
Gradle release build with "JAVA_HOME is set to an invalid directory".
Re-running flutter config --jdk-dir on every devbox shell init keeps
it correct for this project.

devbox.json

@@ -18,6 +18,7 @@
   },
   "shell": {
     "init_hook": [
+      "flutter config --jdk-dir \"$(dirname \"$(dirname \"$(command -v java)\")\")\" > /dev/null",
       "flutter --version",
       "if command -v android >/dev/null 2>&1; then export ANDROID_HOME=$(command -v android | sed -E 's|(.*libexec/android-sdk).*|\\1|'); fi",
       "if [ -n \"$ANDROID_HOME\" ]; then export PATH=$ANDROID_HOME/cmdline-tools/latest/bin:$PATH; fi"