Remove the need to read the opam file inside the archive on a version pin

[kit-ty-kate]

In #4580 @rjbou fixed a bug in the case we're reading the opam file inside the archive but the opam file is invalid. It seems that we should not look into the archive at all. We should review that at some point.

[dra27]

Linking in the original issue #3651 and the fix #3726

[dra27]

This Dockerfile also demonstrates the original problem:

FROM ocaml/opam:alpine-3.12-ocaml-4.07
RUN git clone https://github.com/ocaml/opam.git
WORKDIR opam
RUN git checkout 2.0.3 && sed -i -e 's/\.ua//' src_ext/Makefile.sources
RUN eval $(opam env) && ./configure && make lib-ext && make
RUN sudo cp opam /usr/bin/opam
RUN sudo apk add m4
RUN opam --version
RUN rm -rf ~/.opam && opam init -ya 'git+https://github.com/ocaml/opam-repository.git#7484a9dd' -c ocaml-base-compiler.4.07.1
RUN opam repo add coq-core-dev https://coq.inria.fr/opam/core-dev
RUN opam pin add coq 8.9.dev -y
RUN opam exec -- coqc --version
RUN opam pin add coq 8.8.2 -y
RUN opam exec -- coqc --version

The 2.0.4 tag executes correctly.