raco exe: fix handling of unwritable source executable

Make the executable writable while it's patched for new content.

Closes PR 15008

Merge to v6.2

(cherry picked from commit 5affb68478)

racket/collects/compiler/embed.rkt

@@ -1358,6 +1358,7 @@
                                   (when (file-exists? dest)
                                     (delete-file dest)))
                               (raise x))])
+        (define old-perms (ensure-writable dest-exe))
         (when (and (eq? 'macosx (system-type))
                    (not unix-starter?))
           (let ([m (or (assq 'framework-root aux)
@@ -1654,7 +1655,8 @@
                      (let ([m (and (eq? 'windows (system-type))
                                    (assq 'subsystem aux))])
                        (when m
-                         (set-subsystem dest-exe (cdr m)))))])))))))))
+                         (set-subsystem dest-exe (cdr m)))))]))))
+          (done-writable dest-exe old-perms))))))
 
 ;; For Mac OS X GRacket, the actual executable is deep inside the
 ;;  nominal executable bundle
@@ -1664,3 +1666,20 @@
     [(list? p) (map mac-mred-collects-path-adjust p)]
     [(relative-path? p) (build-path 'up 'up 'up p)]
     [else p]))
+
+;; Returns #f (no change needed) or old permissions
+(define (ensure-writable dest-exe)
+  (cond
+   [(member 'write (file-or-directory-permissions dest-exe))
+    ;; No change needed
+    #f]
+   [else
+    (define old-perms
+      (file-or-directory-permissions dest-exe 'bits))
+    (file-or-directory-permissions dest-exe (bitwise-ior old-perms #o200))
+    old-perms]))
+
+;; Restores old permissions (if not #f)
+(define (done-writable dest-exe old-perms)
+  (when old-perms
+    (file-or-directory-permissions dest-exe old-perms)))