respect the machine file binary overrides, even if it doesn't exist

If someone specifies a binary in a machine file, but the resulting
prog.found() is false because it doesn't actually exist on disk, then
the user was probably trying to disable finding that program. But
find_program() currently doesn't distinguish between a machine file
lookup returning a not-found program, and returning a dummy program
because there's no entry at all.

Explicitly check for a dummy program, rather than checking if the
program was found, before deciding whether to discard the lookup results
and continue trying other program lookup methods.

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