Write to bench.out-tmp only once

Write to bench.out-tmp only once

Appending benchmark program output on every run could result in a case
where the benchmark run was cancelled, resulting in a partially
written file.  This file gets used again on the next run, resulting in
results being appended to old results.

It could have been possible to remove the file before every benchmark
run, but it is easier to just write the output to bench.out-tmp only
once.