Fix: Added guard for jdk auto download

[codeholic-0]

link_linux: download JDK automatically if missing

Trying to MasonInstall java-language-server on Neovim gave the error:

         ◍ java-language-server
      ▼ Displaying full log
        Cloning git repository "https://github.com/georgewfraser/java-language-server.git"…
        Cloning into '.'...
        From https://github.com/georgewfraser/java-language-server
         * tag               v0.2.39    -> FETCH_HEAD
        + ./scripts/link_linux.sh
        Error: This JDK does not support linking from the current run-time image
        spawn: bash failed with exit code 2 and signal 0. Cloning into '.'...
        From https://github.com/georgewfraser/java-language-server
         * tag               v0.2.39    -> FETCH_HEAD
        + ./scripts/link_linux.sh

Because Mason is running ./script/link_linux.sh which doesn't auto download the jdk before linking and crahses
to fix added :

if [ ! -d "$JAVA_HOME" ]; then
    ./scripts/download_linux_jdk.sh
fi

Added a guard that downloads the jdk automatically if the target
directory doesn't exist, making the script self-contained.