ci: Disable wasi test except on stable.

This results in an error when building with 1.64 despite `AsFd`
having been added in 1.63. It was fixed in 1.66:

rust-lang/rust#103306

.github/workflows/ci.yaml

@@ -57,7 +57,10 @@ jobs:
         if: ${{ matrix.rust == 'stable' }}
         run: cargo clippy -- -D warnings
 
+      # This fails on 1.64, but works on 1.66 and later.
+      # https://github.com/rust-lang/rust/issues/103306
       - name: Test run targeting WASI
+        if: ${{ matrix.rust == 'stable' }}
         run: |
           curl https://wasmtime.dev/install.sh -sSf | bash
           source ~/.bashrc