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.