Fix standalone script failures due to USE_CACHE.

Change If27ecd51f386035df7baba3386d545b2c36fb126 allowed re-use of
cached assets, however, it assumed USE_CACHE was set externally.
We now apply a default in each of the three layers.

Change-Id: I1cd4a34de765a2ea93373ce416b40f8aee377ba7