[ci.yml] Don't run tests on push

We require that all commits to the `main` branch go via pull requests,
and that pull requests can only be merged once all tests pass. Thus,
there's no need to run tests whenever commits are pushed; it just wastes
cycles.

.github/workflows/ci.yml

@@ -1,7 +1,6 @@
 name: Build & Tests
 
 on:
-  push:
   pull_request:
 
 env: