Make Windows Python setup more standard

[matthew-brett]

I was using my own Python install, because I was testing on a local
machine, but it's true that we should use the GHA Python setup tools for
this.

Thanks @tylerjereddy for the comment:
#15563 (review)

I agree too that the post-install is a little gruesome. In fact we have
the same stuff implemented in Python in the dev.py script. It might
be better to refactor that out into something in the _build_tools
directory, and run it as single Python command instead. dev.py
could import the same code.

Reference issue

What does this implement/fix?

Additional information

[tylerjereddy]

LGTM, the CircleCI timeout is not related.

The new windows job is already a few minutes faster than the old build system Windows jobs. I imagine we'll have better traction to evaluate the discrepancy in build times on different operating systems/compilers later on (I think I already saw the use of a ninja profiler in chrome in one of the PRs/tools, which I've used at work as well..).

[tylerjereddy]

thanks