CCM nodes share the same environment variable map breaking upgrade tests

In node.py __environment_variables is generally always set with a map that is passed in from cluster.py so it is shared between nodes and if nodes modify the map, such as in start when updating the Java version then when get_env runs it will overwrite the Java version that is selected by update_java_version.

This results in nodetool drain failing when upgrading from 3.11 to 4.0 in some of the upgrade tests because after the first node upgrades to 4.0 it's not longer possible for the subsequent nodes to select a Java version that isn't 11 because it's overridden by __environment_variables.

I'm not even 100% clear on why the code in start should update __environment_variables at all if we calculate the correct java version on every invocation of other tools.