Store opam repo hash at a single place

[cuihtlauac]

The Opam switch is using a repo pinned at a certain hash. This freezes the history and turns opam update into a noop. However, the hash is written in several files.

>git grep b457e9f3d6
.github/workflows/ci.yml:            pin: git+https://github.com/ocaml/opam-repository#b457e9f3d6
.github/workflows/dummy.yml:            pin: git+https://github.com/ocaml/opam-repository#b457e9f3d6
.github/workflows/scrape.yml:            pin: git+https://github.com/ocaml/opam-repository#b457e9f3d6
Dockerfile:RUN cd opam-repository && git checkout -b freeze b457e9f3d6 && opam update
Makefile:       opam switch create . 4.14.1 --no-install --repos pin=git+https://github.com/ocaml/opam-repository#b457e9f3d6

It should be stored once.