feat: update pip conditionally

[Seiikatsu]

Hello! 👋

Thanks for providing this nice action, it makes our CI-Life much easier! :)

We have a use-case where we use custom docker images which already have tools pre-installed. One of those tools is pip which was installed using the package manager of the distroy, therefore when this action runs pip install --upgrade pip it will fail our pipeline.

I added a new variable to disable this behaviour and skip the pip upgrade. The default behaviour is untouched, means it shouldn't break existing workflows / pipelines.