Add Store::buildPathsWithResults()

This function is like buildPaths(), except that it returns a vector of
BuildResults containing the exact statuses and output paths of each
derivation / substitution. This is convenient for functions like
Installable::build(), because they then don't need to do another
series of calls to get the outputs of CA derivations. It's also a
precondition to impure derivations, where we *can't* query the output
of those derivations since they're not stored in the Nix database.

Note that PathSubstitutionGoal can now also return a BuildStatus.

src/libstore/build/goal.hh

@@ -2,6 +2,7 @@
 
 #include "types.hh"
 #include "store-api.hh"
+#include "build-result.hh"
 
 namespace nix {
 
@@ -55,6 +56,9 @@ struct Goal : public std::enable_shared_from_this<Goal>
     /* Whether the goal is finished. */
     ExitCode exitCode;
 
+    /* Build result. */
+    BuildResult buildResult;
+
     /* Exception containing an error message, if any. */
     std::optional<Error> ex;