Increase solver timeout

Trying to fix ocurrent#354.
Recreates ocurrent#342 (for which the branch has been deleted).

If this timeout is long enough, it will address the timeouts on opam
2.0, 2.1, and 2.2, since this env var has been supported at least since 2.0:
https://github.com/ocaml/opam/blob/7d4a0f2e0fefe748efbd7358775b1a0bd8267544/src/solver/opamSolverConfig.ml#L131-L132

This can increase the build time of some tests, but I think it should be
worth it if it can give proper results:

- Only few builds (~3) per PR run on these older opam versions.
- We are wasting a lot of cumulative preson time having to check the CI
- results for PR that should be green if the CI wasn't giving these
  (arguably) false negatives.

lib/opam_build.ml

@@ -125,7 +125,7 @@ let setup_repository ?(local=false) ~variant ~for_docker ~opam_version () =
   run "uname -rs && opam exec -- ocaml -version && opam --version" ::
   env "OPAMDOWNLOADJOBS" "1" :: (* Try to avoid github spam detection *)
   env "OPAMERRLOGLEN" "0" :: (* Show the whole log if it fails *)
-  env "OPAMSOLVERTIMEOUT" "500" :: (* Increase timeout. Poor mccs is doing its best *)
+  env "OPAMSOLVERTIMEOUT" "1000" :: (* Increase timeout. Poor mccs is doing its best *)
   env "OPAMPRECISETRACKING" "1" :: (* Mitigate https://github.com/ocaml/opam/issues/3997 *)
   env "CI" "true" :: env "OPAM_REPO_CI" "true" :: (* Advertise CI for test frameworks *)
   [