nixpkgs/elvish: use pwd for pushd

pwd ensures we get the absolute path

.config/nixpkgs/elvish/lib/pushd.elv

@@ -1,9 +1,9 @@
 var dir-stack = [$pwd]
 
-fn on-chdir {|dir|
+fn on-chdir {|_dir|
   var last = $dir-stack[0]
-  if (not-eq $dir $last) {
-    set dir-stack = [ $dir $@dir-stack ]
+  if (not-eq $pwd $last) {
+    set dir-stack = [ $pwd $@dir-stack ]
   }
 }