V8_COMPILE_CACHE_CACHE_DIR default doesn't match README

[lightmare]

README says it defaults to <os.tmpdir>/v8-compile-cache-<v8.version>
whereas the code defaults to <os.tmpdir>/v8-compile-cache-<UID>/<v8.version>

Aside from that, I think it'd make more sense for the variable to only specify the base dir, and for <v8.version> to always be appended,
i.e. given V8_COMPILE_CACHE_CACHE_DIR=/my/v8cache
files would go in /my/v8cache/<v8.version>/*.BLOB etc.