Remove old action.yml file and update release workflow (rust-lang#1881)

Remove old action file since it's in a new repo now. I also updated our release workflow to check the new repo instead of this old file.

Author: celinval

.github/workflows/release.yml

@@ -19,6 +19,12 @@ jobs:
       - name: Checkout code
         uses: actions/checkout@v2
 
+      - name: Check action code
+        uses: actions/checkout@v2
+        with:
+            repository: model-checking/kani-github-action
+            path: kani-action
+
       - name: Get version
         run: |
           # pkgid is something like file:///home/ubuntu/kani#kani-verifier:0.1.0
@@ -37,8 +43,9 @@ jobs:
             exit 1
           fi
           # Check the action script's tag.
-          if ! grep -F "image: docker://ghcr.io/model-checking/kani-ubuntu-20.04:${{ env.TAG_VERSION }}" action.yml; then
-            echo "Git tag ${{env.TAG_VERSION}} did not match version in action.yml"
+          if ! grep -F "KANI_VERSION=\"${{ env.TAG_VERSION }}" kani-action/action.yml; then
+            echo "Git tag ${{env.TAG_VERSION}} did not match version in action.yml. Found:"
+            grep -F "KANI_VERSION=" kani-action/action.yml
             exit 1;
           fi