Build log is attached, but in short:

Warning: JAVA_HOME environment variable is not set.
Error: Could not find or load main class org.codehaus.plexus.classworlds.launcher.Launcher
Exception in thread "main" java.lang.RuntimeException: Error running command, return status !=0: [sh, C:\test server\apache-maven-3.2.5/bin/mvn, install:install-file, -Dfile=work\mapped.323675e4.jar, -Dpackaging=jar, -DgroupId=org.spigotmc, -DartifactId=minecraft-server, -Dversion=1.12-SNAPSHOT]
at org.spigotmc.builder.Builder.runProcess(Builder.java:561)
at org.spigotmc.builder.Builder.main(Builder.java:328)

I keep hitting this stumbling block, despite tinkering with the environment variables.