Revert "fix: run postinstall script explicitly with bash (#4116)"

This reverts commit b32b4ed.

We are reverting this because we found out that while this fixes the postinstall
on Windows, it breaks it on mac and other devices.  See:
#3874 (comment)

ci/build/build-release.sh

@@ -49,7 +49,7 @@ bundle_code_server() {
   {
     "commit": "$(git rev-parse HEAD)",
     "scripts": {
-      "postinstall": "bash ./postinstall.sh"
+      "postinstall": "./postinstall.sh"
     }
   }
 EOF