Install native build toolchain in website PR validation workflow (#4753)

The website build runs javadoc-gen.sh, which invokes `mvn install` on
bookkeeper-server and transitively requires native-io. After #4738
migrated native-io from C to Rust, building native-io requires
cargo-zigbuild, Rust toolchains, and Zig — none of which were installed
in this workflow, causing every website PR check to fail with
`error: no such command: zigbuild`.

Reuse the existing setup-native-build composite action so the website
workflow matches the other workflows that build native-io.

Author: merlimat

.github/workflows/website-pr-validation.yml

@@ -43,6 +43,8 @@ jobs:
           distribution: 'temurin'
           java-version: 17
 
+      - name: Set up native build toolchain
+        uses: ./.github/actions/setup-native-build
 
       - name: Setup NodeJS
         uses: actions/setup-node@v6