[ci] benchmarks: suppress wrong exit code (#5344)

* [ci] benchmarks: suppress wrong exit code

The script uses '!' to prevent set -e failures,
however a $? of `0` becomes exit code 1 if it is the last command ran.

This `true` makes sure that $? is 0 when we reach the end,
instead of carrying over a failure which would otherwise cause
the whole CI job to abort.

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