Message10073

We can see that in JarInstaller.java on line 179 that the installer change current directory to path/to/install/bin before execute path/to/install/bin/jython.

If we go under path/to/instal/bin just run directly jyhon is sufficient.

https://hg.python.org/jython/file/77e0e7c87bd1/installer/src/java/org/python/util/install/JarInstaller.java#l179

The source:
String launcher = bindir.resolve("jython").toString();
String command[] = new String[]{ launcher, "-m", "ensurepip"};
ChildProcess childProcess = new ChildProcess(command);
childProcess.setCWD(bindir);
errorCode = childProcess.run();

I suggest to make command like that:
String command[] = new String[] { "jython", "-m", "ensurepip" };