We default to workspace build for build-webkit since https://commits.webkit.org/251501@main. We should do the same for build-jsc and build-webgpu, which are the two users of `BuildSubproject.pm` logic that calls into Make.

This does not strictly need to be done before switching all Make-based builds to default to workspace mode. However, doing this first means that:

1) We'll override the default workspace behavior to ensure that `build-jsc` and friends always use the open-source WebKit.xcworkspace
2) We avoid redundant `make` invocations once Make switches to workspace mode by default