buildPaths(): Add an evalStore argument

With this, we don't have to copy the entire .drv closure to the
destination store ahead of time (or at all). Instead, buildPaths()
reads .drv files from the eval store and copies inputSrcs to the
destination store if it needs to build a derivation.

Issue #5025.

src/libcmd/installables.cc

@@ -766,7 +766,7 @@ BuiltPaths build(ref<Store> evalStore, ref<Store> store, Realise mode,
     if (mode == Realise::Nothing)
         printMissing(store, pathsToBuild, lvlError);
     else if (mode == Realise::Outputs)
-        store->buildPaths(pathsToBuild, bMode);
+        store->buildPaths(pathsToBuild, bMode, evalStore);
 
     return getBuiltPaths(evalStore, store, pathsToBuild);
 }