Tracking the file descriptor leaks issues

[moyodiallo]

This is for tracking the current file descriptor leaks issues:

• [X] the pipes are not close after a child process finished its job and solved by this PR #39.
• [x] Using the git-unix during the update of opam-repository and that could be solved by this PR https://github.com/mirage/ocaml-git/pull/590
• [x] When we are getting the oldest opam-repository commit after doing a solve, we can end-up with lot pipes opened at same time. This is PR #47 that is supposed to solve it.
• [X] close the all unused file descriptors opened by git-unix #49

cc @tmcgilchrist

[moyodiallo]

After the merge of this PR #49, we can revert https://github.com/ocurrent/ocaml-ci/pull/742 (upgrade solver/solver-ci and ocaml-ci/solver-ci submodule https://github.com/ocurrent/ocaml-ci/pull/757).

This is already stress tested :+1:

@tmcgilchrist @maiste @mtelvers