Add enable_android_sdk_build: true to pull_request.yml

[marcprux]

Following the precedent of #261, swift-algorithms should be built for Android as one of the officially supported platforms on CI (added in swiftlang/github-workflows#172) 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