buildserver: delete corrupt files from the cache

In order to make CI and other automation easier, delete any corrupt files
from the cache if they fail the SHA-256 check.

https://jenkins.debian.net/view/reproducible/job/reproducible_setup_fdroid_build_environment_profitbricks3/112/console
1 job for standalone-vagrantfile in 14 minutes and 48 seconds (queued for 148 minutes and 34 seconds)
Status Job ID Name Coverage
  Test
passed #2217478
test

00:14:48