[system test] Run DOCs STs generator only in install goal not verify (during tests)

[see-quick]

Type of change

• Enhancement / new feature
• Refactoring

Description

This PR changes the way how we run DOCs STs generator. Currently we run it also when one runs

mvn verify...  (i.e, during tests)

and we do not want it. Instead of this we want to run it during build process

mvn clean install

Checklist

• Make sure all tests pass

[scholzj]

And do we ever do mvn install in the Build pipeline for system testsM

[see-quick]

And do we ever do mvn install in the Build pipeline for system testsM

Yes we do

make -C systemtest java_install
make[1]: Entering directory '/home/vsts/work/1/s/systemtest'
echo "Installing root pom ..."
Installing root pom ...
mvn -B -DskipTests -Dmaven.javadoc.skip=true install -f ../pom.xml -N