Always run CI (#453)

I find the current `bors` workflow a bit tedious. Most of the time, I summon `bors` to see the CI results (see e.g. #438). Given that most `CI` tests are quick (< 10mins), we can always run them by default. 

The most time-consuming `IntegrationTests` is still run by `bors` to avoid excessive CI runs.

Author: yebai

.github/workflows/CI.yml

@@ -9,6 +9,9 @@ on:
       - trying
       # Build the master branch.
       - master
+  pull_request:
+    branches:
+      - master
 
 jobs:
   test: