Fix newline issue in reproduce_ci_env.py

MR !2598 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.
5 jobs for reproduce-ci-fix