Update mxnet workflow to Ubuntu-20.04

Signed-off-by: Beat Buesser <[email protected]>

Author: Beat Buesser

.github/workflows/ci-mxnet.yml

@@ -23,7 +23,7 @@ on:
 
 jobs:
   test:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     strategy:
       fail-fast: false
       matrix: