CI: use OCaml 5.1.x instead of 5.0

Except on Windows, as I'm unsure why there was a Windows exception
in the first place.

Author: xavierleroy

.github/workflows/build.yml

@@ -22,9 +22,9 @@ jobs:
           - 4.14.x
         include:
           - os: ubuntu-latest
-            ocaml-compiler: 5.0.x
+            ocaml-compiler: 5.1.x
           - os: macos-latest
-            ocaml-compiler: 5.0.x
+            ocaml-compiler: 5.1.x
           - os: windows-latest
             ocaml-compiler: ocaml.5.0.0,ocaml-option-mingw