Benchable should not uncompress builds for every iteration

[AlexDaniel]

See this: https://github.com/perl6/whateverable/blob/9b747bc43e61f340295e1fc497f0535d460554d3/bin/Benchable.p6#L43-L44

run-snippet will then call run-smth: https://github.com/perl6/whateverable/blob/9b747bc43e61f340295e1fc497f0535d460554d3/lib/Whateverable.pm6#L405-L407

run-smth is basically a wrapper that takes a block and wraps some logic around it to uncompress and then delete builds. So the for ^ITERATIONS logic should be inside that block, otherwise it will spend a lot of time just uncompressing stuff.