do not run on each push

The master branch is already restricted from pushes so the code can reach the master branch only through a pull requested. We don't want to the same work twice.

Also removes (the commented) windows line, we will return it back, once we will have time for that, see also #512

Author: ivg

.github/workflows/main.yml

@@ -1,7 +1,6 @@
 name: Main workflow
 
 on:
-  - push
   - pull_request
 
 jobs:
@@ -12,7 +11,6 @@ jobs:
         os:
           - ubuntu-latest
           - macos-latest
-          #- windows-latest
         ocaml-version:
           - 4.09.1
           - 4.08.1