Windows: add TBB (and MinGW DLLs) to load path

[WardBrian]

This is in response to #117. It isn't a full "fix" (arbitrary user configs could still run into similar problems and need to set their own %PATH%s correctly), but it attempts to be a bit friendlier.

This is similar to code in cmdstanpy. The main difference is (in Python) it also uses os.add_dll_directory. This appears to have fixed the issue which was preventing us from running our Python tests on Windows in Github Actions, so I've re-enabled those.

I'll do a follow on PR which tries to split the Windows CI in to separate jobs per-interface.

Note: This PR does not change the R interface, since R doesn't (yet) have any handling of where BridgeStan is installed (see #100)

[WardBrian]

It turns out that whether the os.add_dll_directory call is necessary is OS (windows only), version (behaves differently in 3.8 and 3.10), AND distribution (anaconda python doesn’t need it, python from python.org does) specific.

I have confirmed that it never hurts in situations where it isn’t required, but that explains why I could never get the tests to run in CI but could locally.

[roualdes]

Just my one question, which you likely already thought through. I just want to make sure I'm on the same page. Thanks.

[roualdes]

The logic here seems slightly different, as compared to the logic in Python. So this comment is just meant to double check on that difference.

Under Julia, if where.exe can find tbb.dll, then the Julia ENV["PATH"] will be able to successfully find tbb.dll, is this correct? And it's only if where.exe can't find tbb.dll that we need to add tbb.dll to the path?

[WardBrian]

Yes, Python specifically does not look in PATH for dependent DLLs, which as far as I can tell all other languages do.