Can't build openjdk:jdk 16 and above on aarch64 without hitting TasksMax

[Brennan Vincent]

See https://lists.gnu.org/archive/html/guix-devel/2016-04/msg00608.html
for historical context.

I have to bump this value locally in order to build openjdk@16:jdk
because the build requires more than 10000 PIDs.

Perusing the output of ps suggests that the build process is leaking a
huge number of unreaped bash processes. The number keeps growing
monotonically until the build is finished.