Scratch Obeya stages on Windows without a \tmp folder

The script that stages a throwaway Obeya for demos and checks kept its folders under a fixed /tmp. On Windows that meant \tmp on the current drive, and on a fresh machine without that folder, staging failed before it began. On Windows the folders now go into the system temp folder (%TEMP%). macOS and Linux keep /tmp/obeya-scratch-<port>, as before. Every build now stages a scratch Obeya on Windows, Linux and macOS, so a break on any of them shows up at once.