fix(melange): build time benchmarks

[anmonteiro]

These seem to be failing with:

[WARNING] Failed checks on pupilfirst package definition from source at
git+file:///home/runner/work/dune/dune/pupilfirst#master: warning 74:
Field 'pin-depends' contains packages that are neither in 'depends' nor
in 'depopts': "reason-react-ppx"

not entirely sure that the pin-depends are being respected.

[anmonteiro]

OK it sounds like this is related to OPAM 2.4 not respecting pins for some reason when you run opam install . --deps-only.

[anmonteiro]

This was a regression in OPAM: ocaml/opam#6611

Thanks @kit-ty-kate for the help on this one.