github actions: The build-gpu action always fails

[texadactyl]

https://github.com/UCBerkeleySETI/turbo_seti/blob/master/.github/workflows/push_docker.yml

"No runner matching the specified labels was found: nvidia-gpu"

I realise that nvidia-gpu is a runner that we are waiting for (yes?).

Suggestion: Comment out the build-gpu step until we have a reasonable chance of it succeeding.

[luigifcruz]

Looks good to me.