[CI] Clean up ci_build.sh wrapper script for legibility and clarity

[hcho3]

The tests/ci_build/ci_build.sh script used to be a small wrapper that ran a given command inside a Docker image. Over time, the wrapper script has become highly complex. I recently made a mistake that increased the number of times Docker images were re-built from scratch (#6305), and that one-line mistake cost us dearly, burning precious CI budget ~1.5x faster.

The wrapper script must be cleaned up for legibility and clarity.

[hcho3]

Suggestion from @trivialfis: Use Python instead of Bash, for robust manipulation of strings.