Scope down --derivation to just the commands that use it

Per the old FIXME, this flag was on too many commands, and mostly
ignored. Now it is just on the commands where it actually has an effect.

Per https://github.com/NixOS/nix/issues/7261, I would still like to get
rid of it entirely, but that is a separate project. This change should
be good with or without doing that.

src/libcmd/command.hh

@@ -96,9 +96,6 @@ struct SourceExprCommand : virtual Args, MixFlakeOptions
     std::optional<std::string> expr;
     bool readOnlyMode = false;
 
-    // FIXME: move this; not all commands (e.g. 'nix run') use it.
-    OperateOn operateOn = OperateOn::Output;
-
     SourceExprCommand(bool supportReadOnlyMode = false);
 
     std::vector<std::shared_ptr<Installable>> parseInstallables(
@@ -153,8 +150,15 @@ private:
     std::string _installable{"."};
 };
 
+struct MixOperateOnOptions : virtual Args
+{
+    OperateOn operateOn = OperateOn::Output;
+
+    MixOperateOnOptions();
+};
+
 /* A command that operates on zero or more store paths. */
-struct BuiltPathsCommand : public InstallablesCommand
+struct BuiltPathsCommand : InstallablesCommand, virtual MixOperateOnOptions
 {
 private: