Install nightly toolchain

.github/workflows/validate-pr.yaml

@@ -2,11 +2,15 @@ name: Validate Pull Request
 on: push
 
 jobs:
-  build:
+  test:
     name: Run Tests
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@master
+    - uses: actions-rs/toolchain@v1
+      with:
+        toolchain: nightly
+        override: true
     - uses: actions-rs/cargo@v1
       with:
         command: test