Fix parallel download errors when solution has multiple projects that use WasmComponent.Sdk

[jsturtevant]

componentize-dotnet/src/WasmComponent.Sdk/build/WasmComponent.Sdk.targets

Lines 48 to 57 in b3a97cb

	<!--
	This is not ideal because if your solution has multiple projects that use WasmComponent.Sdk, then if you
	build in parallel in CI where your machine doesn't already have wasi-sdk/emsdk, then it may try to download
	and extract the SDKs multiple times in parallel to the same disk location, which may cause it to fail.
	The only reason this doesn't happen in this repo is that it explicitly runs the PrepareWasmSdks task before
	building other projects.
	For a proper fix, consider implementing an MSBuild task in C# that obtains wasi-sdk/emsdk, and uses a mutex
	so that only one flow executes at a time, with others blocking until it's done.
	-->