Build ocamlfind in AppVeyor tests

[dra27]

Travis (and Cygwin) both test an opam switch by installing ocamlfind. This is the very last part of #3260 bringing parity to the main testsuite (and leaving "only" opam-rt to go).

This cannot be merged until the %{exe}% variable in #3900 is merged and some hygiene patches are made to the ocamlfind package in opam-repository so that a1e0fa8 can be blown away.

[XVilka]

Could be updated I think? Opam and ocamlfind both had changed since. Also there are warnings still:

The following actions will be performed:
  - install conf-m4   1     [required by ocamlfind]
  - install ocamlfind 1.8.0
===== 2 to install =====
<><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><><><>
-> installed conf-m4.1
-> retrieved ocamlfind.1.8.0  (http://download.camlcity.org/download/findlib-1.8.0.tar.gz)
[WARNING] .install file is missing .exe extension for src/findlib/ocamlfind
[WARNING] Automatically adding .exe to C:\projects\opam\.opam\default\.opam-switch\build\ocamlfind.1.8.0\src\findlib\ocamlfind.exe
[WARNING] .install file is missing .exe extension for src/findlib/ocamlfind_opt
[WARNING] Automatically adding .exe to C:\projects\opam\.opam\default\.opam-switch\build\ocamlfind.1.8.0\src\findlib\ocamlfind_opt.exe
[WARNING] C:\projects\opam\.opam\default\bin\safe_camlp4 is a script; the command won't be available
#=== ERROR while installing ocamlfind.1.8.0 ===================================#
Strict mode is enabled - previous warnings considered fatal
<><> Error report <><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>
+- The following actions failed
| - install ocamlfind 1.8.0
+- 
+- The following changes have been performed
| - install conf-m4 1
+-

[dra27]

GitHub actions install the lwt package as a test on a all platforms.