On Windows, the tmpDir function is currently using a buffer on the stack
to convert the WTF16-encoded environment variable value "TMP" to utf8
and then returns it as a slice...but that stack buffer is no longer valid
when the function returns. This was causing the "image load...temporary
file" test to fail on Windows.
I've updated the function to take an allocator but it only uses
the allocator on Windows. No allocation is needed on other platforms
because they return environment variables that are already utf8 (ascii)
encoded, and the OS pre-allocates all environment variables in the process.
To keep the conditional that determines when allocation is required, I
added the `freeTmpDir` function.
