[workflows] Temporarily switch to stable for CI

`cargo +nightly test` crashes with nightly da80ed070.  This will prevent lints
from running on push, however, and should be changed back once nightly is
fixed.

Author: mshroyer

.github/workflows/ci.yml

@@ -21,5 +21,7 @@ jobs:
     uses: ./.github/workflows/build-test.yml
     with:
       os: ubuntu-latest
-      toolchain: nightly
+      # TODO: Switch back to nightly, so that lints run, once `cargo +nightly
+      # test` no longer crashes rustc
+      toolchain: stable
       web: true