Misc Windows fixes

[t-kalinowski]

I tested out py_require() on Windows and noticed a few issues:

1. uv writes progress updates to stdout on Windows. This interfered with our ability to resolve the path to the ephemeral python, since we expected stdout to contain just the filepath.

   To fix, I refactored uv_get_or_create() to pass through both stderr and stdout from uv, and write the python executable path to a temporary file instead of stdout.

2. The codepath that installed uv produced a file named 'Out-Null' in the current working directory.

   To fix, I refactored the system2() call to use stdout=FALSE

3. The codepath that installed uv didn't seem to handle paths or mixed / in the file correctly.

   Rather than think too hard about how quoting needs to happen for cmd.exe calling powershell, I refactored the installer to pass filepaths via an envvar, and also, use shortPathName().