[minor] fix dune build
[minor] fix dune build

The script would previously use `grep 'ARCH=' Makefile.config` to
locate the ARCH-setting line of the configuration. But now the
configuration looks like this

    ### Set ARCH=none if your machine is not supported
    ARCH=amd64

and the 'grep' call trips up on the comment. This commit uses '^ARCH'
instead. Hopefully that is portable to other system's `grep` command.

gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml