Shebang parser: add virtual destructor

Fixes:

    warning: destructor called on non-final 'nix::ParseUnquoted' that has virtual functions but non-virtual destructor [-Wdelete-non-abstract-non-virtual-dtor]

src/libutil/args.cc

@@ -97,6 +97,8 @@ struct Parser {
     virtual void operator()(std::shared_ptr<Parser> & state, Strings & r) = 0;
 
     Parser(std::string_view s) : remaining(s) {};
+
+    virtual ~Parser() { };
 };
 
 struct ParseQuoted : public Parser {