Make the GitHub actions that Bors relies on run on the staging and trying branches.

Bors expects GitHub to run the workflows automatically anytime it pushes to the `staging` and `trying` branches. Without this change, Bors times out waiting for the workflows to run.

.github/workflows/ci.yml

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 744ea69..ad0738e 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -1,7 +1,15 @@
 # This workflow provides automated testing. It builds and runs tests on each PR.
 
 name: ci
-on: pull_request
+
+# We want to run CI on all pull requests. Additionally, Bors needs workflows to
+# run on the `staging` and `trying` branches.
+on:
+  pull_request:
+  push:
+    branches:
+      - staging
+      - trying
 
 jobs:
   ci:

.github/workflows/size-diff.yml

diff --git a/.github/workflows/size-diff.yml b/.github/workflows/size-diff.yml
index 6f8c7d0..2c3b597 100644
--- a/.github/workflows/size-diff.yml
+++ b/.github/workflows/size-diff.yml
@@ -2,7 +2,15 @@
 # request is created or modified.
 
 name: size-diff
-on: pull_request
+
+# We want to run this on all pull requests. Additionally, Bors needs workflows
+# to run on the `staging` and `trying` branches to block merges on them.
+on:
+  pull_request:
+  push:
+    branches:
+      - staging
+      - trying
 
 jobs:
   size-diff: