GeezerVison

Removes the nix dev image from .github/workflows/dogfood.yaml on release/2.34, aligning it with release/2.35 and later (["22.04", "26.04"]). Tracked in ENT-214.

Why

build_image (nix) fails on every release/2.34 PR that triggers the dogfood workflow (e.g. #30811):

error: cannot download google-chrome-stable_143.0.7499.169-1_amd64.deb from any mirror (HTTP 404)

The nix dev image includes google-chrome from the nixos-25.05 nixpkgs pinned in this branch's flake.lock. Google only serves recent Chrome builds, so the pinned .deb is gone. main removed the nix image in #26022 after 2.34 was cut, so only this branch still builds it. Bumping nixpkgs instead would change many packages on an old release branch, and the Chrome pin would go stale again.

Changes

.github/workflows/dogfood.yaml only, mirroring the workflow part of #26022:

• Drop nix from the build_image matrix.
• Remove the nix-only steps: Nix install, cache-nix-action, nix build .#dev_image, and the codercom/oss-dogfood-nix push.
• Drop the matrix.image-version != 'nix' guards, which are now always true.
• Drop the flake.nix / flake.lock path triggers, since no job in this workflow uses the flake anymore.
• Update comments that mentioned the nix image.

Not touched, to keep the backport small: flake.nix (dev_image output), nix/docker.nix, and the nix image option in dogfood/coder/main.tf. None of them is used by CI after this change.

Notes

• Analyze PR for Documentation Updates Needed fails here for an unrelated reason (outdated agents-chat-action pin, fixed by ci: bump agents-chat-action to v0.5.1 in doc-check (backport 2.34) #30850 / ENT-212).
• chromatic fails on all release/2.34 PRs ("No app with code ... found"), also unrelated.