[Snek] [keith-packard/snek] dfb6db: .github: Force pip install of pynsist

[Keith Packard]

  Branch: refs/heads/main
  Home:   https://github.com/keith-packard/snek
  Commit: dfb6dba8e210dd571840e9b8425f08271939c028
      https://github.com/keith-packard/snek/commit/dfb6dba8e210dd571840e9b8425f08271939c028
  Author: Keith Packard <keithp at keithp.com>
  Date:   2023-09-26 (Tue, 26 Sep 2023)

  Changed paths:
    M .github/Dockerfile

  Log Message:
  -----------
  .github: Force pip install of pynsist

Even though we're *gasp* not doing everything in a virtual env.

Signed-off-by: Keith Packard <keithp at keithp.com>