Hi again,

Nix commit 55586527 (June 2013) changed the semantics of
‘preferLocalBuild’ from “avoid offloading this derivation” to “avoid
offloading *or substituting* this derivation” (see
<http://bugs.gnu.org/18747>.)

This patch introduces a new special key, ‘substitution’, to specify
whether a derivation should be substituted.  ‘preferLocalBuild’ is kept,
but its initial semantics is restored.

As a consequence, existing uses of ‘preferLocalBuild’ must be audited to
check what exactly is intended.

WDYT?

Thanks,
Ludo’.