Don't print values in magenta

This fixes the opening bracket of lists/attrsets being printed in
magenta, unlike the closing bracket.

NixOS#9753 (comment)

src/libexpr/print.cc

@@ -511,4 +511,15 @@ std::ostream & operator<<(std::ostream & output, const ValuePrinter & printer)
     return output;
 }
 
+/**
+ * `ValuePrinter` does its own ANSI formatting, so we don't color it
+ * magenta.
+ */
+template<>
+hintformat & hintformat::operator%(const ValuePrinter & value)
+{
+        fmt % value;
+        return *this;
+}
+
 }