ci/eval: accept nix directly

Previously we were taking nixVersions and this made external use from
the Lix repo's CI annoying.

We should probably also test other nix versions than stable (i.e. also
latest and Lix), but this involves writing GitHub Actions about it and
maybe not running it on every single PR. Future work.

ci/default.nix

@@ -76,7 +76,13 @@ in
   inherit pkgs fmt;
   requestReviews = pkgs.callPackage ./request-reviews { };
   codeownersValidator = pkgs.callPackage ./codeowners-validator { };
-  eval = pkgs.callPackage ./eval { };
+
+  # FIXME(lf-): it might be useful to test other Nix implementations
+  # (nixVersions.stable and Lix) here somehow at some point to ensure we don't
+  # have eval divergence.
+  eval = pkgs.callPackage ./eval {
+    nix = pkgs.nixVersions.latest;
+  };
 
   # CI jobs
   lib-tests = import ../lib/tests/release.nix { inherit pkgs; };

ci/eval/default.nix

@@ -1,15 +1,23 @@
+# Evaluates all the accessible paths in nixpkgs.
+# *This only builds on Linux* since it requires the Linux sandbox isolation to
+# be able to write in various places while evaluating inside the sandbox.
+#
+# This file is used by nixpkgs CI (see .github/workflows/eval.yml) as well as
+# being used directly as an entry point in Lix's CI (in `flake.nix` in the Lix
+# repo).
+#
+# If you know you are doing a breaking API change, please ping the nixpkgs CI
+# maintainers and the Lix maintainers (`nix eval -f . lib.teams.lix`).
 {
   callPackage,
   lib,
   runCommand,
   writeShellScript,
-  writeText,
   symlinkJoin,
   time,
   procps,
-  nixVersions,
+  nix,
   jq,
-  python3,
 }:
 
 let
@@ -31,8 +39,6 @@ let
       );
     };
 
-  nix = nixVersions.latest;
-
   supportedSystems = builtins.fromJSON (builtins.readFile ../supportedSystems.json);
 
   attrpathsSuperset =

lib/tests/test-with-nix.nix

@@ -3,9 +3,10 @@
 
   IMPORTANT:
   This is used by the github.com/NixOS/nix CI.
+  This is used by Lix's CI (see flake.nix in the Lix repo).
 
   Try not to change the interface of this file, or if you need to, ping the
-  Nix maintainers for help. Thank you!
+  Nix AND Lix maintainers (`nix eval -f . lib.teams.lix`) for help. Thank you!
 */
 {
   pkgs,