DTC_JAVA_HOME should supersede JAVA_HOME

[ascheman]

Describe the bug

If JAVA_HOME is set (e.g., by sdkman), and DTC_JAVA_HOME is explicitely set or computed by DTC, the latter should be used.

To Reproduce
Steps to reproduce the behavior:

1. Install local (DTC) Java: dtcw install java -> results in ~/.doctoolchain/jdk
2. Set JAVA_HOME to a different directory (or simply use SDKMAN
3. Run ./dtcw local generateSite -> Will use the Java from ${JAVA_HOME} instead of the one installed by step one.
4. On many systems/architectures this might work perfectly without problems. On Apple Silicon this might result in a Gradle error Can't load library: ... (since JBake still uses an x86_64 native library).