github: migrate actions

.github/main.workflow

-action "Auto Assign" {
-  uses = "ipfs/auto-assign@v1.0.0"
-  secrets = ["GITHUB_TOKEN"]
-}
-workflow "Add reviewers/assignees to Pull Requests" {
-  on = "pull_request"
-  resolves = "Auto Assign"
-}

.github/workflows/pull_request.yml

+on: pull_request
+name: Add reviewers/assignees to Pull Requests
+jobs:
+  autoAssign:
+    name: Auto Assign
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@master
+    - name: Auto Assign
+      uses: ipfs/auto-assign@v1.0.0
+      env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}