CI: Include stdlib build for Wasm in the Linux buildbot (#77949)
* **Explanation**: "Swift Test Linux Platform" jobs on ci.swift.org required as PR checks should cross-compile and run tests for WebAssembly. Effectively, what was previously an optional `@swift-ci test webassembly` trigger should become an implicit and required part of PR testing to prevent occasional regressions in Wasm and WASI support.
* **Scope**: Only affects the WASI platform
* **Risk**: Low. It increases job run time by ~10% and this platform has active maintainers.
* **Testing**: stdlib test suite is now enabled.
* **Issue**: N/A
* **Reviewer**: @shahmishal
* **Main branch PR**: https://github.com/swiftlang/swift/pull/77872
