Prior to this commit, the build tool plugins set the environment
variable BP_JAVA_VERSION when invoking the CNB builder to set the
version of the JDK/JRE that the builder should use in the created
image.

With CNB API 0.3, the convention changed the name of this environment
variable to BP_JVM_VERSION. This commit updates the build tool
plugins to match the newer convention.

See gh-21273