Escape ${ in strings when printing Nix expressions

Otherwise the result of the printing can't be parsed back correctly by
Nix (because the unescaped `${` will be parsed as the begining of an
anti-quotation).

Fix #3989

src/libexpr/eval.cc

@@ -87,6 +87,7 @@ static void printValue(std::ostream & str, std::set<const Value *> & active, con
             else if (*i == '\n') str << "\\n";
             else if (*i == '\r') str << "\\r";
             else if (*i == '\t') str << "\\t";
+            else if (*i == '$' && *(i+1) == '{') str << "\\" << *i;
             else str << *i;
         str << "\"";
         break;