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
Status Name Job ID Coverage
  External
passed cdash-buildbot

passed vtk-m-dragnipur-osx-shared-debug+32bit_ids+64bit_coords+clang+tbb

00:12:46

passed vtk-m-dragnipur-osx-static-release+64bit_coords+clang+tbb

00:00:16

passed vtk-m-osheim-windows-shared-debug+ninja-vs-2017+tbb

00:06:50

passed vtk-m-osheim-windows-static-release+ninja-vs-2017+tbb

00:33:47