Makefile: opam install the lock file

[reynir]

According to this comment we need to opam install the lockfile:

tarides/opam-monorepo#378 (comment)

But in the code we install the .opam file (and not the .opam.locked file):

https://github.com/mirage/mirage/blob/main/lib/functoria/makefile.ml#L138-L140

CC @TheLortex @samoht

[TheLortex]

Indeed we should install the .opam.locked file

[reynir]

Reopening this as #1462 was reverted. It seems given the current state of opam-monorepo we should not opam install the lock file but also we should(?!) it would be good to have clarified the expectations of opam-monorepo and what the workflow should look like.

[hannesm]

When installing the lockfile, we ended up in #1476 which adds lots of pins to the current opam switch. An option would be to --ignore-pin-depends -- but in the end, I'm more interested in which concrete issue "using the lockfile" would solve.