Update Rust new AllocRef API checking workflow

Author: luojia65

.github/workflows/rust.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        rust: [nightly-2020-02-01, nightly-2020-02-03, nightly-2020-03-01, nightly-2020-03-04]
+        rust: [nightly-2020-02-01, nightly-2020-02-03, nightly-2020-03-01, nightly-2020-03-04, nightly-2020-03-20]
 
     steps:
     - uses: actions/checkout@v2