sanitycheck: don't override environment for USE_CCACHE

Many users set USE_CCACHE=1 in their envionment. Sanitycheck was
disregarding that and placing USE_CCACHE=0 in the generated
Makefile unless --ccache was added to the command line every time
it was invoked.

Issue: ZEP-1663
Change-Id: Id3c1379f5039d4d2f4a3ae01d2ec8d0dd216f523
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>