make_manifest_spiff should not assume TMPDIR is set

On my Ubuntu box, `TMPDIR` is not set, which causes `make_manifest_spiff` to try to create a directory under `/` which it doesn't have permission to do. As there is no error checking in the script it carries on regardless, but fails to work.

This PR ensures that `temp_dir` is set to something valid even if `TMPDIR` is not set

I don't think there is a sensible system-wide default for TMPDIR that is properly cross platform - so have used `BOSH_LITE_HOME/tmp` as the fallback default.