Strip UNC prefix when setting working directory (#1277)
## Summary
For PEP 517 builds, the current working directory needs to be set to the
directory of the source distribution. It turns out that on Windows, if
you use a UNC path for the working directory, then relative paths are
interpreted relative to the root of the current drive
([source](https://www.fileside.app/blog/2023-03-17_windows-file-paths/#paths-relative-to-the-root-of-the-current-drive)).
So, when builds attempted to resolve relative paths, they always
errored...
This PR ensures that we remove the UNC prefix when setting the current
working directory.
Closes #1238.
## Test Plan
I tested this on my Windows machine by installing `ujson` with
`--no-binary ujson`. (I don't want to add that specific test, since it's
really slow to build.)
