Install META files for the standard OCaml distribution atomically

[kit-ty-kate]

dune has the base packages builtin so ocamlfind is not required. However, when a META file is present dune reverts back to it. In this case, a race condition can occure on packages that for example uses threads.posix and is built at the same time as ocamlfind is installed. Such package would fail on some rare circonstances if the META file was in the middle of being copied

[kit-ty-kate]

@gerdstolpmann any chance this could be looked at? I encountered the issue in CI again

#=== ERROR while compiling ssl.0.5.10 =========================================#
# context              2.1.0~rc2 | linux/x86_64 | ocaml-base-compiler.4.12.0 | file:///src
# path                 ~/.opam/4.12/.opam-switch/build/ssl.0.5.10
# command              ~/.opam/opam-init/hooks/sandbox.sh build dune build -p ssl -j 47
# exit-code            1
# env-file             ~/.opam/log/ssl-20-839b19.env
# output-file          ~/.opam/log/ssl-20-839b19.out
### output ###
#     ocamlopt src/config/discover.exe (exit 2)
# (cd _build/default && /home/opam/.opam/4.12/bin/ocamlopt.opt -w -40 -g -o src/config/discover.exe /home/opam/.opam/4.12/lib/csexp/csexp.cmxa /home/opam/.opam/4.12/lib/result/result.cmxa /home/opam/.opam/4.12/lib/dune-configurator/configurator.cmxa src/config/.discover.eobjs/native/dune__exe__Discover.cmx)
# File "_none_", line 1:
# Error: No implementations provided for the following modules:
#          Unix referenced from /home/opam/.opam/4.12/lib/dune-configurator/configurator.cmxa(Configurator__V1)

Full log: https://opam.ci.ocaml.org/github/ocaml/opam-repository/commit/efa2ae6bdaf2fd0fa458a9ef2266267922064428/variant/opam-2.1,compilers,4.12,parsexp.v0.14.1,revdeps,sihl-cache.1.0.0~rc1