Accessing some of the stdenvs would cause an infinite recursion. We can
continue to rely on base stdenv and also bail if any field other than
mkDerivation is used. All the stdenvs supported have a subset of the
attrs of base stdenv, so this is safe.
The mock pkgs set passed `{}` as the value for unknown args; if a
package def tried to evaluate one, this can raise an error that can't be
caught. Replacing the default value with `throw ""` enables bailing out
if one of the fake args is accessed.
Since evaluating pkgs is expensive, this speeds up evaluation of flakes
with `packages.default` by attempting to get the name using a pkgs set
containing only mocked builders that just return the name.
