Revert to install opam from debian

This is the version deployed in production (I think?) and the current
(outdated) installed version in the docker image is making
swh-loader-core tests fail.

bookworm-multipython/Dockerfile

@@ -123,6 +123,7 @@ RUN --mount=type=cache,id=apt-cache,target=/var/cache/apt \
   mercurial \
   myrepos \
   nodejs \
+  opam \
   pkg-config \
   postgresql-15 \
   postgresql-client-15 \
@@ -180,16 +181,6 @@ RUN mkdir -p /home/${user}/.local/bin && \
 ENV GEM_HOME=/home/${user}/.local/rubygems
 RUN gem install bibliothecary
 
-ARG opam_version=2.0.10
-
-# install opam
-RUN curl -sL https://raw.githubusercontent.com/ocaml/opam/master/shell/install.sh -o /tmp/opam-install && \
-  cd /home/${user}/.local/bin && \
-  bash /tmp/opam-install --version ${opam_version} --download-only && \
-  chmod +x opam-* && \
-  ln -s opam-* opam && \
-  rm /tmp/opam-install
-
 # Install rustup and the default rust toolchain
 
 ENV CARGO_HOME="/home/${user}/.cargo"