Update URL component source cache to use configured root

[itowlson]

The URL component source cache PR (#1088) included the comment:

"TODO: when spin up integrates running an app from OCI, pass the configured cache root to this function. For now, use the default cache directory." (https://github.com/fermyon/spin/pull/1088/files/4d99bcf842d7e3e1166db8a43a438613e33b0090#r1098922615)

Now that spin up work has been done, we should check back on this and, if necessary, update URL source caching to use the configured cache root if there is one.

cc @fibonacci1729

[itowlson]

I'm pretty sure this is done. #2377 is building on what's already there and it sure as heck looks like it's done. Closing: reopen if I'm wrong.