Fix flake input completion for InstallablesCommands

Defers completion of flake inputs until the whole command line is parsed
so that we know what flakes we need to complete the inputs of.

Previously, `nix build flake --update-input <Tab>` always behaved like
`nix build . --update-input <Tab>`.

src/libcmd/command.hh

@@ -77,12 +77,16 @@ struct MixFlakeOptions : virtual Args, EvalCommand
 {
     flake::LockFlags lockFlags;
 
+    std::optional<std::string> needsFlakeInputCompletion = {};
+
     MixFlakeOptions();
 
     virtual std::vector<std::string> getFlakesForCompletion()
     { return {}; }
 
     void completeFlakeInput(std::string_view prefix);
+
+    void completionHook() override;
 };
 
 struct SourceExprCommand : virtual Args, MixFlakeOptions