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; };