CI: Avoid duplicate tests on pull requests

Seems testing on "push" events is enough.
Before this commit, pushing and opening a PR
caused the same commit to be tested twice.

.github/workflows/ci.yml

@@ -1,6 +1,15 @@
 name: CI
 
-on: [push, pull_request]
+on:
+  push:
+    branches:
+      - '*'
+    tags-ignore:
+      - '*'
+    paths:
+      - '.github/workflows/ci.yml'
+      - 'package_control/**/*.py'
+  workflow_dispatch:
 
 jobs:
   test-all: