Update GitHub actions to v2

Author: nspark

.github/workflows/main.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v1
+    - uses: actions/checkout@v2
     - name: Install dependencies
       run: |
         sudo apt-get update
@@ -18,7 +18,7 @@ jobs:
     - name: Rename PDF
       run: mv main_spec.pdf openshmem-draft-${{ github.event.pull_request.head.sha }}.pdf
     - name: Upload PDF artifact
-      uses: actions/upload-artifact@v1
+      uses: actions/upload-artifact@v2
       with:
         name: openshmem-draft-${{ github.event.pull_request.head.sha }}
         path: openshmem-draft-${{ github.event.pull_request.head.sha }}.pdf