actions docker_push_image: Update nix 2.13.3 -> 2.20.3

[roberth]

Motivation

#9546 (comment)

Context

Priorities and Process

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[roberth]

Clearly there's no CI for this job, so I'll test by merging. See master commit status in a couple min...

[thufschmitt]

The job on master broke, but for an unrelated reason (and broke later, so the PR fixed the bug it was supposed to fix):

The push refers to repository [ghcr.io/nixos/nix]
tag does not exist: ghcr.io/nixos/nix:master
Error: Process completed with exit code 1.