[FEATURE] Add environment variable to override default location for cache and log files.

[tneer]

• Jake puts log and cache files in the ${HOME}/.ossindex directory. There are some cases where this isn't optimal. For example, in a CI environment with multiple executors on the same agent there may be multiple simultaneous jake invocations. It would be helpful in this situation if there was a JAKE_HOME or JAKE_WORKDIR environment variable that could be set to override the default location.

• This is mostly useful for CI but there may be other automation cases where the home directory for a daemon user isn't writable.

• Something like environ.get('JAKE_WORKDIR', Path.home()) should do the trick. I think there are a few places that change would need to be made. I don't quite know how to test it since my Python knowledge is not great.

cc @bhamail / @DarthHater