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>
