make the dune 2.8.1 package use the system dune

[sadiqj]

It looks like while dune 2.6.0 uses the system dune, the recently introduced 2.8.1 does not.

This fixes breakage when running sandmark on trunk.

[shakthimaan]

The following steps were used with Sandmark (ed00c5fb88e261c5b712389bd73ae1c27ed3e0ad) to get the 4.12 stock build running:

sudo docker run --name test -it ocurrent/opam:ubuntu-20.10-ocaml-4.12
sudo apt-get update && sudo apt-get -y install wget pkg-config libgmp-dev m4 libdw-dev jq python3-pip
pip3 install intervaltree
sudo chown -R opam .
eval $(opam env)
opam update
opam install dune.2.7.1
export ITER=1
export OPAM_DISABLE_SANDBOXING=true
git clone https://github.com/ocaml-bench/sandmark
cd sandmark
TAG='"run_in_ci"' make run_config_filtered.json
RUN_CONFIG_JSON=run_config_filtered.json make ocaml-versions/4.12.0+stock.bench

From inside the docker:

opam@aa9952cacc2c:~/sandmark$ dune --version
2.7.1

opam@aa9952cacc2c:~/sandmark$ ls _results/ -lh
total 12K
-rw-r--r-- 1 opam opam 9.1K May 26 13:59 4.12.0+stock_1.orun.summary.bench

[shakthimaan]

Sandmark has been updated to use dune.2.9.0 with the following merge #251. Hence, closing this PR.