Add enable_wasm_sdk_build: true to pull_request.yml

[MaxDesiatov]

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

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• [N/A] I've updated the documentation if necessary

[MaxDesiatov]

@swift-ci test

[natecook1000]

Thanks for adding this!