nix app: Search for installable in the 'apps' output

I.e. you can write

  $ nix app blender-bin:blender_2_80

which is equivalent to

  $ nix app blender-bin:apps.blender_2_80

1 files changed, 5 insertions, 0 deletions

diff --git a/src/nix/run.cc b/src/nix/run.cc
index 00a682832..62aae12f6 100644
--- a/src/nix/run.cc
+++ b/src/nix/run.cc
@@ -225,6 +225,11 @@ struct CmdApp : InstallableCommand, RunCommon
         return {"defaultApp"};
     }
 
+    Strings getDefaultFlakeAttrPathPrefixes() override
+    {
+        return {"apps."};
+    }
+
     void run(ref<Store> store) override
     {
         auto state = getEvalState();