Unpin nightly toolchain (#2161)

Seeing what's left on CI to fix

Author: alexcrichton

azure-pipelines.yml

@@ -76,7 +76,7 @@ jobs:
     steps:
       - template: ci/azure-install-rust.yml
         parameters:
-          toolchain: nightly-2020-05-24
+          toolchain: nightly
       - template: ci/azure-install-node.yml
       - script: cargo test --target wasm32-unknown-unknown --features nightly --test wasm
 
@@ -276,7 +276,7 @@ jobs:
     steps:
       - template: ci/azure-install-rust.yml
         parameters:
-          toolchain: nightly-2020-05-24
+          toolchain: nightly
       - script: cargo doc --no-deps --features 'nightly serde-serialize'
         displayName: "Document wasm-bindgen"
       - script: cargo doc --no-deps --manifest-path crates/js-sys/Cargo.toml