expander: fix another local-expand problem with 'module-begin

Alexis's repair, and as she notes, forcing a `post-expansion` context
value in the core `#%module-begin` expander may allow a simplification
in "definition-context.rkt". But it's not immediately obvious, so save
that potential improvement for later.

Relevant to #2118

pkgs/racket-test-core/tests/racket/macro.rktl

@@ -2206,6 +2206,34 @@
     (#%module-begin
      (m))))
 
+(module test-for-scope-specific-binding/nested racket/base
+
+  (module l racket/base
+    (require (for-syntax racket/base racket/syntax)
+             syntax/parse/define)
+
+    (provide expand-in-modbeg m
+             (all-from-out racket/base))
+
+    (define-syntax-parser expand-in-modbeg
+      [(_ form ...)
+       (local-expand #'(#%plain-module-begin form ...)
+                     'module-begin
+                     '())
+       #'(void)])
+
+    (define-syntax-parser m
+      [(_)
+       #:with x (generate-temporary)
+       #'(begin
+           (define x #f)
+           (begin-for-syntax #'x))]))
+
+  (module c (submod ".." l)
+    (let ()
+      (expand-in-modbeg
+       (m)))))
+
 ;; ----------------------------------------
 
 (report-errs)

racket/src/expander/expand/module.rkt

@@ -252,7 +252,12 @@
                                                           [compiled-submodules (make-hasheq)]
                                                           [compiled-module-box (box #f)]
                                                           [defined-syms (make-hasheq)])
-                                    (module-begin-k s ctx)))]))
+                                    (module-begin-k s ctx)))]
+                               ;; Also, force `post-expansion` to be right, in case 'module-begin
+                               ;; module is triggered within some other mode; a correct value
+                               ;; for `post-expansion` is important to getting scopes right.
+                               [post-expansion #:parent root-expand-context
+                                               (lambda (s) (add-scope s inside-scope))]))
 
      ;; In case `#%module-begin` expansion is forced on syntax that
      ;; that wasn't already introduced into the mdoule's inside scope,

racket/src/expander/expand/root-expand-context.rkt

@@ -61,7 +61,7 @@
   (cond
     [(not pe) s]
     [(shifted-multi-scope? pe) (push-scope s pe)]
-    [(pair? pe) (syntax-add-shifts (add-scope s (car pe)) (cdr pe))]
+    [(pair? pe) (syntax-add-shifts (push-scope s (car pe)) (cdr pe))]
     [else (pe s)]))
 
 (define (post-expansion-scope pe)