Stop running CI on push (#429)

Running CI on push and PR is wasteful: for PRs from
`wasmtime/wasmtime-rb` branches, we end up running CI twice.

The downside of this is that we can no longer have a CI run without a
PR. But now that CI has draft PRs, it's not really a concern.

Author: jbourassa

.github/workflows/ci.yml

@@ -3,9 +3,6 @@ name: CI
 
 on:
   workflow_dispatch:
-  push:
-    branches: ["**"]
-    tags-ignore: ["v**"] # Skip CI for releases
   pull_request:
 
 concurrency: