[wasm] Remove "UsingBlazorOrWasmSdk" condition from emscripten workload detection

[maraf]

• Similar change to [wasm] Remove _UsingBlazorOrWasmSdk condition from browser-wasm workload detection #52250. Syncs workload manifest logic with [browser] Library mode without workload runtime#122607 and [browser] Library mode without workload runtime#122607 (comment), which enables browser-wasm library mode without requiring the workload.
• Remove SelfContained=true from WorkloadManifest. We set it for WasmSDK in https://github.com/dotnet/sdk/blob/main/src/WasmSdk/Sdk/Sdk.props#L19 and there is no requirement to have it set just for RID=browser-wasm lib

[Copilot]

Pull request overview

This PR removes the Blazor/WebAssembly SDK condition from Emscripten workload detection logic, syncing with runtime changes that enable browser-wasm library mode without requiring the workload. This is a companion change to PR #52250 which made a similar update to the Mono.Toolchain manifest.

Changes:

• Simplified the UsingBrowserRuntimeWorkload condition to only require the workload when AOT compilation is explicitly enabled, removing the check that forced workload usage for non-Blazor/WebAssembly SDK scenarios
• Added a guard to prevent overriding previously set UsingBrowserRuntimeWorkload values