Description:
Allow to add another toolchain only by a subsequent setup-java - only toolchains.xml must be updated with the additional JDK setup and the JDK specific JAVA_HOME_<jdk version> must be set.
No update of settings.xml, no add to PATH, no update of JAVA_HOME.

Justification:
In my scenario I run a first setup-java configuring settings.xml and setting up my JAVA_HOME JDK.
Then I determine a probable additionally required toolchain version from the compile target setting of my Maven projects POM.
If this is requiring a different JDK, then I am running another setup-java for this toolchain.
In this case I want to get the toolchains.xml being extended only and to preserve JAVA_HOME, PATH and settings.xml from my first setup-java.
Currently I am working around this requirement by a step preserving JAVA_HOME, PATH and settings.xml before the setup-java step and restoring those after that step. This is working fine so far.
Another config property for setup-java like e.g. add-toolchain-only would save those efforts!

Are you willing to submit a PR?
Yes - cannot promise a timeline yet.