ci: try to simplify ab setup

.github/workflows/assets.yml

@@ -24,12 +24,15 @@ jobs:
       with:
         submodules: recursive
     - run: git submodule absorbgitdirs
-    - run: |
-        mkdir -p ~/.ssh
-        echo "${{ secrets.id_rsa_ab }}" > ~/.ssh/id_rsa
-        chmod 600 ~/.ssh/id_rsa
-        ssh-keyscan -t rsa github.com
-        git clone git@github.com:lichess-org/ab.git && echo "::set-env name=LILA_AB_FILE::$PWD/ab/src/ab.js" || true
+    - uses: actions/checkout@v2
+      with:
+        repository: lichess-org/ab
+        ssh-key: ${{ secrets.id_rsa_ab }}
+        path: ab
+      continue-on-error: true
+      id: ab
+    - run: echo "::set-env name=LILA_AB_FILE::$GITHUB_WORKSPACE/ab/src/ab.js"
+      if: steps.ab.outcome == 'success'
     - uses: actions/cache@v1
       with:
         path: ~/.cache/yarn