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.
