cerbero: Don't extract if already extracted in fetch

Some recipes need to extract the source tree in order to know what else needs to be fetched. At present, this is true for Cargo recipes where we need to run cargo vendor to fetch sources for offline build, and for Meson recipes that require subprojects with wrap files, where we need to download sources by reading the wrap files.

This is especially important for Cargo recipes, because cargo vendor can take a dozen minutes or more on Windows. This should speed up the MSVC CI by 15-20 minutes (when running under full load).