Makefile: Set OPAMCLI to 2.0

[Alasdair]

This removes the warning about opam config var. Setting OPAMCLI in this way is the correct thing to do if we want to continue supporting opam 2.0.

If we decide to require opam 2.1+ then all opam var invocations should become opam --cli=2.1 var, as per the opam CLI versioning spec:

https://github.com/ocaml/opam/wiki/Spec-for-opam-CLI-versioning

[github-actions]

Test Results

712 tests  ±0   712 ✔️ ±0   0s ⏱️ ±0s
    6 suites ±0       0 💤 ±0 
    1 files   ±0       0 ❌ ±0 

Results for commit 545000d. ± Comparison against base commit 153f983.

[billmcspadden-riscv]

lgtm

[Alasdair]

Hmm, is anyone else still seeing these warnings? I'm thinking I maybe tricked myself into thinking this fixed it somehow. I think maybe you actually need to set it in every subshell like:

$(shell OPAMCLI=2.0 opam config var ...)

to get rid of all the warnings

[bacam]

I'm still getting them. Experimenting a little, it appears that the shell function isn't affected by export, at least in GNU make 4.3. It's probably worth keeping the assignment, though, and using $(shell OPAMCLI=$(OPAMCLI) ...) so that there's a consistent setting everywhere, including inside scripts.

On a related note, we should use := for all the uses of shell to ensure that they're only run once. That speeds up all make invocations.