[CI] Enable Wasm SDK build in swift-foundation

[jrflat]

Now that swiftlang/github-workflows supports building with the Swift Wasm SDK in swiftlang/github-workflows#142, we can enable the job in swift-foundation to prevent WASI build regressions.

[jrflat]

Swift Wasm SDK nightly-main snapshot releases are generally pretty regular (though a bit of a gap from 6/22-7/17). I think we should be OK to use it here:

curl -s https://www.swift.org/api/v1/install/dev/main/wasm-sdk.json | jq . | grep "date"
    "date": "2025-08-02 10:10:00 -0600",
    "date": "2025-07-28 10:10:00 -0600",
    "date": "2025-07-26 10:10:00 -0600",
    "date": "2025-07-24 10:10:00 -0600",
    "date": "2025-07-23 10:10:00 -0600",
    "date": "2025-07-18 10:10:00 -0600",
    "date": "2025-07-17 10:10:00 -0600",
    "date": "2025-06-22 10:10:00 -0600",
    "date": "2025-06-21 10:10:00 -0600",
    "date": "2025-06-20 10:10:00 -0600",
    "date": "2025-06-17 10:10:00 -0600",
    "date": "2025-06-12 10:10:00 -0600",
    "date": "2025-06-09 10:10:00 -0600",
    "date": "2025-06-03 10:10:00 -0600",

[jrflat]

@swift-ci please test