Remove Python setup and fix multiline prerequisites output

[colluca]

• Extend test to generate a list with multiple prerequisites.
• Fix multiline prerequisite output, by converting newlines to spaces. This is in general more useful, as this file list would be used as an argument to a command-line tool.
• Remove Python setup to avoid conflicts with Python installations on e.g. workflows using a Docker container with a preinstalled Python toolchain.
• Improve pip invocation and explicitly specify path to requirements.txt file in action's repository.
• Sort prerequisites to ensure consistency