fix(nix): don't report a broken install for a freshly mounted /nix volume (#2601)

[mikeland73]

Summary

Fixes #2601.

When running Devbox in a container (e.g. Docker or Kubernetes) with /nix mounted as a persistent volume, the volume starts out effectively empty. Devbox is supposed to install Nix into it, but instead it fails with:

Error: We found a /nix directory but nix binary is not in your PATH and we were not able to find it in the usual locations. Your nix installation might be broken. [...]

Root cause

EnsureNixInstalled decided whether /nix already contained an installation by checking whether the directory was non-empty:

if dirExistsAndIsNotEmpty("/nix") { ... return "broken installation" error ... }

A truly empty /nix already falls through to the installer, but a freshly mounted volume is usually not truly empty: ext4 (and other filesystems) create a lost+found directory at the root of the mount. So /nix looks non-empty, Devbox assumes a pre-existing (broken) install, and refuses to install Nix.

Fix

Detect an existing installation by ignoring known filesystem cruft (lost+found, .DS_Store) rather than treating any non-empty /nix as installed. A /nix that contains only those entries is now treated as a fresh install target, so Devbox proceeds to install Nix as expected. A /nix that contains real content (e.g. a store directory) is still treated as an existing installation, preserving the original "broken install" diagnostics.

The generic dirExistsAndIsNotEmpty helper was only used here, so it's replaced by a /nix-specific nixDirIsInstalled helper.

How was it tested?

• go test ./internal/nix/ -run TestNixDirIsInstalled -v — new table-driven test covering: empty dir, dir with files, dir with a nix store, dir with hidden files, dir with only lost+found (the regression case → treated as not installed), dir with lost+found + store (→ installed), and non-existent dir. All pass.
• go build ./... / go vet ./internal/nix/ / gofmt clean.

(Pre-existing TestConfigIsUserTrusted failures in the package are unrelated — they require a nix binary that isn't present in the CI sandbox and are not touched by this change.)

cc @ascknx (issue reporter)

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