Add additional builds on 1.50 and 1.51 validating addr_of codepath

Author: dtolnay

.github/workflows/ci.yml

@@ -12,7 +12,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        rust: [nightly, beta, stable]
+        rust: [nightly, beta, stable, 1.51.0, 1.50.0]
     steps:
       - uses: actions/checkout@v2
       - uses: dtolnay/rust-toolchain@master