feat(flake.nix): switch dogfood dev image to buildNixShellImage from dockerTools (#16223)

Replace Depot build action with Nix for Nix dogfood image builds

The dogfood Nix image is now built using Nix's native container tooling instead of Depot. This change:

- Adds Nix setup steps to the GitHub Actions workflow
- Removes the Dockerfile.nix in favor of a Nix-native container build
- Updates the flake.nix to support building Docker images
- Introduces a hash file to track Nix-related changes
- Updates the vendorHash for Go dependencies

Change-Id: I4e011fe3a19d9a1375fbfd5223c910e59d66a5d9
Signed-off-by: Thomas Kosiewski <tk@coder.com>

scripts/update-flake.sh

@@ -1,7 +1,7 @@
 #!/usr/bin/env bash
 # Updates SRI hashes for flake.nix.
 
-set -eu
+set -euo pipefail
 
 cd "$(dirname "${BASH_SOURCE[0]}")/.."
 
@@ -37,4 +37,6 @@ echo "protoc-gen-go version: $PROTOC_GEN_GO_REV"
 PROTOC_GEN_GO_SHA256=$(nix-prefetch-git https://github.com/protocolbuffers/protobuf-go --rev "$PROTOC_GEN_GO_REV" | jq -r .hash)
 sed -i "s#\(sha256 = \"\)[^\"]*#\1${PROTOC_GEN_GO_SHA256}#" ./flake.nix
 
+make dogfood/contents/nix.hash
+
 echo "Flake updated successfully!"