Add enable_wasm_sdk_build: true to pull_request.yml

[MaxDesiatov]

swift-atomics should be built for Wasm as one of the officially supported platforms on CI to prevent possible future regressions.

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've verified that my change does not break any existing tests.
• [N/A] I've updated the documentation if necessary.

[lorentey]

Nice! Thank you Max