Switch UI tests to beta toolchain

Should ideally help reduce breakage!

Author: alexcrichton

azure-pipelines.yml

@@ -107,7 +107,7 @@ jobs:
     steps:
       - template: ci/azure-install-rust.yml
         parameters:
-          toolchain: nightly
+          toolchain: beta
       - template: ci/azure-install-node.yml
       - template: ci/azure-install-sccache.yml
       - script: cargo test -p ui-tests