to predict how much disk space a package will require.
* Compute the output / closure size using the info stored in the
Nix database (rather than doing a slow "du").
3 environment variables are important:
TWITTER_USER
TWITTER_PASS
HYDRA_BUILD_BASEURL
- twitter notification is off when TWITTER_USER and TWITTER_PASS are not defined
- if HYDRA_BUILD_BASEURL is not defined, no URL is put in the twitter messages