In Nix itself, the new 'BuildResult' type is returned by the new
'buildDerivation' method, which we don't have and need.
* nix/libstore/build.cc (Goal)[cancel]: Remove.
[timeOut]: New pure virtual method.
(DerivationGoal)[result]: New field.
[cancel]: Remove.
[timedOut, getResult, done]: New methods.
(DerivationGoal::cancel): Remove.
(DerivationGoal::timedOut): New method.
(DerivationGoal::haveDerivation): Call 'done' instead of 'amDone'.
(DerivationGoal::outputsSubstituted): Ditto.
(DerivationGoal::inputsRealised): Ditto.
(DerivationGoal::buildDone): Ditto.
(DerivationGoal::handleChildOutput): Call 'timedOut' instead of
'cancel'.
(DerivationGoal::done): New method.
(SubstitutionGoal)[cancel]: Remove.
[timedOut]: New method.
(SubstitutionGoal::cancel): Remove.
(SubstitutionGoal::timedOut): New method.
(Worker::waitForInput): Use it.
* nix/libstore/store-api.hh (BuildResult): New struct.
Co-authored-by: Ludovic Courtès <ludo@gnu.org>