ci/eval: fix local full eval

The change to use `builtins.storePath` was good - for when the store
path *is* already part of the nix store. In all my tests so far, that
was already the case, because I was iterating on the solution and the
Eval results stayed the same.

But when this is run on a entirely new commit, these the values for
`afterDir` and `combinedDir` are *not* in the store, yet. As part of
running `eval.full` on a new commit they will be created. `eval.full` is
linked up, so that the values passed around there will actually be
derivations, which might not be realized, yet.

Checking whether the input is a path or not fixes this for both cases.

ci/eval/compare/default.nix

@@ -13,7 +13,12 @@
   byName ? false,
 }:
 let
-  combined = builtins.storePath combinedDir;
+  # Usually we expect a derivation, but when evaluating in multiple separate steps, we pass
+  # nix store paths around. These need to be turned into (fake) derivations again to track
+  # dependencies properly.
+  # We use two steps for evaluation, because we compare results from two different checkouts.
+  # CI additionalls spreads evaluation across multiple workers.
+  combined = if lib.isDerivation combinedDir then combinedDir else lib.toDerivation combinedDir;
 
   /*
     Derivation that computes which packages are affected (added, changed or removed) between two revisions of nixpkgs.

ci/eval/diff.nix

@@ -11,8 +11,13 @@
 }:
 
 let
-  before = builtins.storePath beforeDir;
-  after = builtins.storePath afterDir;
+  # Usually we expect a derivation, but when evaluating in multiple separate steps, we pass
+  # nix store paths around. These need to be turned into (fake) derivations again to track
+  # dependencies properly.
+  # We use two steps for evaluation, because we compare results from two different checkouts.
+  # CI additionalls spreads evaluation across multiple workers.
+  before = if lib.isDerivation beforeDir then beforeDir else lib.toDerivation beforeDir;
+  after = if lib.isDerivation afterDir then afterDir else lib.toDerivation afterDir;
 
   /*
     Computes the key difference between two attrs