Refactor: daemons: Simplify some code in start_child.

cppcheck doesn't understand the use of PCMK__VALGRIND_EXEC here, I think
because it's multiple strings concatenated together, and then wedged
inside a ternary operator.

In general I don't like rearranging code just to please static analysis,
but I don't really like how this code was arranged in the first place.
I think it's a little easier to follow duplicated like this.