‘drvAttrs’. This will simplify the implementation of functions such
as ‘overrideDerivation’ in Nixpkgs, which need to filter out any
added attributes such as outPath.
common attribution so that they're evaluated only once, etc. Note
that the default output is now the first element of the "outputs"
attribute, rather than the first element of the sorted list of
outputs. This seems more user-friendly.
