[Sigmon 143] Use LCG_100_nxcals while executing notebooks

Context

• task:

What was done

• current LCG is sourced in the 'notebooks_exec' CI task (and an alias for python is made)
• 'notebooks_exec' CI task uses gitlab-registry.cern.ch/db/swan-spark-notebooks:latest image

TODO

• other tasks regarding the CI for notebooks (https://its.cern.ch/jira/browse/SIGMON-142 and https://its.cern.ch/jira/browse/SIGMON-141)

QA

• no sonar for notebooks
• no tests because that's the CI improvement

Review objectives

• assessment of code functionality
• assessment of code quality
• identifying potential refactorings / logic common to other projects
• knowledge sharing