fix(nushell): register set-java-home as hook

[lassoColombo]

Problem

The current set-java-home.nu defines asdf_update_java_home but never invokes it and never registers it as a hook. As a result, sourcing the script is a no-op:
JAVA_HOME and JDK_HOME are simply never set.

Proposal

Follow the pattern already established for other shells:

• Make the function an --env function so it can actually mutate the caller's environment
• Register it as a hook so the values track the active java version as the user moves around