ci: set toolchain to 1.81.0 for wasm builds

There's a change in how rust 1.82.0 compiles for wasm targets that
wasm-bindgen needs to account for. Till a new release of that project,
and other dependencies, lets pin to our rust version to 1.81 for wasm
tests.

.github/workflows/ci.yml

@@ -52,6 +52,7 @@ jobs:
 
       - name: rust version
         run: |
+          rustup default 1.81.0
           rustc --version
           cargo --version