Bind cross-buffer buffer-local variable correctly.

Bind cross-buffer buffer-local variable correctly.

This fixes bug#68200.

* lisp/emacs-lisp/bytecomp.el (byte-compile-output-docform):
Note that let-binding a buffer local variable leaves it buffer
local, hence to transfer the binding of
byte-compile-dynamic-docstrings to the output buffer, an
intermediate variable is needed.  Implement this.