git-unix: add upper bound on mirage-crypto-rng dep

[emillon]

Similar to #23322:

#=== ERROR while compiling git-unix.3.12.0 ====================================#
# context              2.2.0~alpha~dev | linux/x86_64 | ocaml-base-compiler.5.0.0 | file:///home/opam/opam-repository
# path                 ~/.opam/5.0/.opam-switch/build/git-unix.3.12.0
# command              ~/.opam/opam-init/hooks/sandbox.sh build dune runtest -p git-unix -j 71 --no-buffer
# exit-code            1
# env-file             ~/.opam/log/git-unix-7-4542c2.env
# output-file          ~/.opam/log/git-unix-7-4542c2.out
### output ###
# File "_none_", line 1:
# Alert ocaml_deprecated_auto_include:
# OCaml's lib directory layout changed in 5.0. The unix subdirectory has been
# automatically added to the search path, but you should add -I +unix to the
# command-line to silence this alert (e.g. by adding unix to the list of
# libraries in your dune file, or adding use_unix to your _tags file for
# ocamlbuild, or using -package unix for ocamlfind).
# File "test/smart/test.ml", line 57, characters 43-45:
# 57 | let () = Mirage_crypto_rng_unix.initialize ()
#                                                 ^^
# Error: This expression has type unit but an expression was expected of type
#          'a Mirage_crypto_rng.generator

[kit-ty-kate]

Thanks

[hannesm]

Thanks a lot.