Use temporary db for benchmarking (#12254)

* Use temporary db for benchmarking

If no db option was given benchmarks shall use temporary database.
Otherwise the test can use locally stored database which maybe
out-of-date causing test to fail.

* nicer syntax

* explanatory comment added

* Update utils/frame/benchmarking-cli/src/lib.rs

Co-authored-by: Bastian Köcher <[email protected]>