Fix newline issue in reproduce_ci_env.py

MR !2598 (merged) introduced some fixes to reproduce_ci_env.py. One of them handled newlines in command lists by separating the command lists by newlines in the Dockerfile (which generated a script). Although this worked for some of the docker containers, I found others, like rhel8, where the newlines were not escaped correctly.

Correct this problem by moving back to separating commands with &&. This requires replacing newlines in commands with &&. That in itself is no problem except that if you have a blank line, you get && &&, which is a syntax error for the shell. So, avoid this by stripping newlines from commands.