Override wasm32 release channel to nightly

This should fix running tests on the emscripten target. In the future
it's expected we'll want to entirely remove this builder since it's a
tier 2 target, but for this initially beta-targeted patch keeping the
changes minimal is preferred.

.github/workflows/ci.yml

@@ -265,8 +265,9 @@ jobs:
             os: ubuntu-20.04-8core-32gb
             env: {}
           - name: wasm32
+            env:
+              RUST_CI_OVERRIDE_RELEASE_CHANNEL: nightly
             os: ubuntu-20.04-8core-32gb
-            env: {}
           - name: x86_64-gnu
             os: ubuntu-20.04-4core-16gb
             env: {}

src/ci/github-actions/ci.yml

@@ -428,6 +428,17 @@ jobs:
             <<: *job-linux-8c
 
           - name: wasm32
+            env:
+              # Running emscripten tests currently requires that we are
+              # building a nightly toolchain. Otherwise, we cannot pass
+              # -Zunstable-options to libtest. Normally we workaround this by
+              # setting RUSTC_BOOTSTRAP in the environment, but that doesn't
+              # work for emscripten as environment variables are not threaded
+              # into the compiled code.
+              #
+              # For more details see:
+              # https://emscripten.org/docs/porting/connecting_cpp_and_javascript/Interacting-with-code.html#environment-variables
+              RUST_CI_OVERRIDE_RELEASE_CHANNEL: nightly
             <<: *job-linux-8c
 
           - name: x86_64-gnu