clarify build instructions
On ubuntu the build instructions did not work for me. What did work is the following

Prerequisites: 
sudo apt-get install git make openjdk-21-jdk
JAVA_HOME="" # to make sure it uses openjdk

Then continue with build instructions as written

Again, this worked for me, but should probably be vetted by others to make sure that it works for them. I especially do not know if the Java home actually needs to be set in the way I've described above or not.