Support running integration tests via alternative clcache invocation
Support running integration tests via alternative clcache invocation

By default, the integration tests will launch clcache via the current
Python interpreter. The newly introduced 'CLCACHE_CMD' environment
variable can be used to customize this behaviour such that I can run the
integration tests via Coverage.py.

webmaster128/clcache,webmaster128/clcache,webmaster128/clcache