Run CI on pull requests

.github/workflows/ci.yml

@@ -1,6 +1,6 @@
 name: Continuous integration
 
-on: [push]
+on: [push, pull_request]
 
 jobs:
   check_pr:
@@ -47,3 +47,11 @@ jobs:
       - uses: ./
         with:
           toolchain: stable
+
+  install_stable_through_rust_toolchain_file:
+    runs-on: ubuntu-latest
+    needs: check_pr
+    steps:
+      - uses: actions/checkout@v1
+      - run: echo "stable" > ./rust-toolchain
+      - uses: ./