build: set open file limit based on how many apps are being processed

build: set open file limit based on how many apps are being processed

When running `fdroid build --all` on a buildserver with thousands of apps,
it was frequently hitting the open file limit.  This increases the open
file limit based on how many apps are being process.  It is doubled to
provide a margin of safety.

There are probably open file leaks which ideally would be fixed, but this
is also useful to make things more resilient to all the random stuff apps
include in their build systems.