nix store delete: don't realise paths

Change-Id: I9566974a3afa70c61a0c8185b4dbd24a6440473f

doc/manual/rl-next/delete-no-realise.md

@@ -0,0 +1,11 @@
+---
+synopsis: "`nix store delete` no longer builds paths"
+cls: [2782]
+category: Fixes
+credits: lheckemann
+---
+
+`nix store delete` no longer realises the installables
+specified. Previously, `nix store delete nixpkgs#hello` would download
+hello only to immediately delete it again. Now, it exits with an error
+if given an installable that isn't in the store.

lix/nix/store-delete.cc

@@ -18,6 +18,7 @@ struct CmdStoreDelete : StorePathsCommand
             .description = "Do not check whether the paths are reachable from a root.",
             .handler = {&options.ignoreLiveness, true}
         });
+        realiseMode = Realise::Nothing;
     }
 
     std::string description() override