lib: eachSystem: inline single-use local variables

Inline single-use local variables to minimize cognitive load while
reading the code, unless they cache computational results.

lib.nix

@@ -29,24 +29,25 @@ let
   # Builds a map from <attr>=value to <attr>.<system>=value for each system.
   eachSystem =
     systems: f:
-    let
+    builtins.foldl'
+      (
         # Merge outputs for each system.
-      op =
         attrs: system:
         let
           ret = f system;
-          op =
+        in
+        builtins.foldl' (
           attrs: key:
           attrs
           // {
             ${key} = (attrs.${key} or { }) // {
               ${system} = ret.${key};
             };
-            };
-        in
-        builtins.foldl' op attrs (builtins.attrNames ret);
-    in
-    builtins.foldl' op { } (
+          }
+        ) attrs (builtins.attrNames ret)
+      )
+      { }
+      (
         systems
         ++
           # Add the current system if the --impure flag is used.