Switch to platform-specific getting of temp file

[iboB]

The perf right now is bottlenecked by the boot of the ruby interpreter. And we do that two times. The first call is to get a temp file name. Instead of doing this, use a platform specific non-ruby way of creating this file: mktemp on Linux and... something else¹ on Windows. It would basically be a 2x perf improvement.

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1: Don't know what yet... something involving %TMP%