github: Split setup action in separate steps per runner

.github/actions/setup/action.yml

@@ -6,17 +6,20 @@ runs:
     with:
       node-version: 17
       cache: 'yarn'
+  - if: runner.os == 'macOS'
+    run: brew install wabt
+    shell: bash
   # We depend on a more recent version, not available in current Ubuntu (20.04).
   # - run: sudo apt-get install wabt
-  - run: |
-      if [ "$RUNNER_OS" == "Linux" ]; then
+  - if: runner.os == 'Linux'
+    run: |
       curl -L -s https://github.com/WebAssembly/wabt/releases/download/1.0.30/wabt-1.0.30-ubuntu.tar.gz | tar xvz -C /tmp
       sudo mv /tmp/wabt-1.0.30/bin/* /usr/local/bin
       sudo mv /tmp/wabt-1.0.30/include/* /usr/local/include
       sudo mv /tmp/wabt-1.0.30/lib/*.a /usr/local/lib
-      else
-        brew install wabt
-      fi
     shell: bash
+  - if: runner.os == 'Windows'
+    run: echo "Not supported yet"
+    shell: cmd
   - run: yarnpkg --pure-lockfile
     shell: bash