Add support for windows runners

[galargh]

Windows runners, as applied from this setup, work but are not yet usable. Below is a minimal list of things to do before the rollout.

Future improvements should definitely include populating hostedtoolcache.

TODO

• [x] Run run.cmd in ephemeral mode (in particular, terminate instance immediately after the job is done)
• [x] Configure docker proxy
  • [x] Ensure docker installation uses provided daemon.json
• [x] Configure go proxy
  • [x] Ensure GOPROXY is set globally
• [x] Install dependencies available on hosted runners (use go-libp2p to find out what's typically in use)
  • [x] bash
• [x] Fix adding to GITHUB_PATH
• [x] Set up directory structure matching hosted runners (current home: C:\actions-runner, expected home: D:\a\.github\.github or D:\a\.github or D:\a)