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

This reverts commit b32b4edf3d1167f87cb3f4aaf1acd3d5378cefb3.

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:
https://github.com/cdr/code-server/issues/3874#issuecomment-925397980