Freeze miri as it's broken on nightly

See rust-lang/rust#70055

Author: jonhoo

azure-pipelines.yml

@@ -49,7 +49,9 @@ jobs:
    steps:
      - template: install-rust.yml@templates
        parameters:
-         rust: nightly
+         # fix nightly since miri breaks there frequently
+         # should probably be bumped occasionally
+         rust: nightly-2020-03-16
          components:
            - miri
      - bash: cargo miri setup