fix broken CI - npm install on CC7 is broken but using rh-nodej6-npm works-...
- the latest CERN CC7 docker image does not allow installation of npm (fails with broken dependencies)
- we used sprindale linux repos to install llvm-toolset-6.0, Version 7 is available from linuxsoft.cern.ch, switch to this version.
- linuxsoft.cern.ch does not provide newer upstream packages from SCLO. Upstream CentOS8 llvm-toolset-11.0 is available, linuxsoft only provides version 7.