On Linux, the default /tmp folder is shared across all users and accessible by them. There are some cases in which we put sensitive information in temp and assume it's fine because on Windows, it is. This doesn't actually fix that assumption, since we're currently waiting for a new API that will be introduced in .NET 7 that will make a folder with appropriate permissions. However, this PR changes all the issues Eric Erhardt identified such that they go through a single code path, so to fix the security issue afterwards just requires changing the one place in our code.

It did occur to me that we may not be able to use that API, in which case I can just write something to make a folder with a random name under temp then tweak its permissions.