Fix continuous integration (#224)

The benchmark requires core_{bench,unix}, but we don't declare them in
opam dependencies because of loops.

core_unix requires ocaml >= 4.08.0.

.github/workflows/main.yml

@@ -16,7 +16,7 @@ jobs:
             #- macos-latest
             #- windows-latest
         ocaml-compiler:
-          - 4.03.x
+          - 4.08.x
           - 4.14.x
     runs-on: ${{ matrix.os }}
     steps:
@@ -27,5 +27,6 @@ jobs:
     - run: opam pin -n .
     - run: opam depext -yt re
     - run: opam install -t . --deps-only
+    - run: opam install -y core_bench core_unix
     - run: opam exec -- dune build
     - run: opam exec -- dune runtest

benchmarks/benchmark.ml

@@ -163,4 +163,4 @@ let benchmarks =
     |> Bench.Test.create_group ~name:"tex gitignore"
   ] @ [http_benches]
 
-let () = Command.run (Bench.make_command benchmarks)
+let () = Command_unix.run (Bench.make_command benchmarks)

benchmarks/dune

@@ -1,3 +1,3 @@
 (executable
- (libraries re threads core_bench)
+ (libraries re threads core_bench core_unix.command_unix)
  (name benchmark))