Update of opam-repository commit to retrieve solo5 0.7.5

[hannesm]

This fixes #166

[palainp]

Is there a way to exclude solo5 before 0.7.5 ? Solo5 is currently not restricted in config.ml.

[hannesm]

Since solo5 is a dependency of ocaml-solo5, and not of the unikernel (and thus installed in the opam switch), we can't easily specify this. What we can do:

• mark old solo5 as available: false in opam-repository
• add a lower bound to the mirage tool in respect to solo5

Neither is optimal. On the flipside, our build process uses the latest opam repository commit, and there the opam solver will pick solo5 0.7.5. I reproduced this built on builds.robur.coop (sorry, there may be DNS issues), and will cut a release with this change.