Simplify the definition of rootFS

It was getting very hard to follow.

Author: Ericson2314

src/libexpr/eval.cc

@@ -226,22 +226,25 @@ EvalState::EvalState(
            */
           {CanonPath(store->storeDir), store->getFSAccessor(settings.pureEval)},
       }))
-    , rootFS(({
-        /* In pure eval mode, we provide a filesystem that only
-           contains the Nix store.
-
-           If we have a chroot store and pure eval is not enabled,
-           use a union accessor to make the chroot store available
-           at its logical location while still having the
-           underlying directory available. This is necessary for
-           instance if we're evaluating a file from the physical
-           /nix/store while using a chroot store. */
-        auto accessor = getFSSourceAccessor();
-
-        auto realStoreDir = dirOf(store->toRealPath(StorePath::dummy));
-        if (settings.pureEval || store->storeDir != realStoreDir) {
-            accessor = settings.pureEval ? storeFS.cast<SourceAccessor>() : makeUnionSourceAccessor({accessor, storeFS});
-        }
+    , rootFS([&] {
+        auto accessor = [&]() -> decltype(rootFS) {
+            /* In pure eval mode, we provide a filesystem that only
+               contains the Nix store. */
+            if (settings.pureEval)
+                return storeFS;
+
+            /* If we have a chroot store and pure eval is not enabled,
+               use a union accessor to make the chroot store available
+               at its logical location while still having the underlying
+               directory available. This is necessary for instance if
+               we're evaluating a file from the physical /nix/store
+               while using a chroot store. */
+            auto realStoreDir = dirOf(store->toRealPath(StorePath::dummy));
+            if (store->storeDir != realStoreDir)
+                return makeUnionSourceAccessor({getFSSourceAccessor(), storeFS});
+
+            return getFSSourceAccessor();
+        }();
 
         /* Apply access control if needed. */
         if (settings.restrictEval || settings.pureEval)
@@ -252,8 +255,8 @@ EvalState::EvalState(
                     throw RestrictedPathError("access to absolute path '%1%' is forbidden %2%", path, modeInformation);
                 });
 
-        accessor;
-    }))
+        return accessor;
+    }())
     , corepkgsFS(make_ref<MemorySourceAccessor>())
     , internalFS(make_ref<MemorySourceAccessor>())
     , derivationInternal{corepkgsFS->addFile(