ci: fix checkout action to checkout branch at current state instead of merging into base branch (#55)

Author: wschurman

.github/workflows/tests.yml

@@ -10,6 +10,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
+      with:
+        ref: ${{ github.event.pull_request.head.sha }}
     - uses: actions/setup-node@v1
       with:
         node-version: 10.x