Increase file open limits on x86_64/i686 builder

Context

This is failing a lot of builds when bst1 runs out of fd's. The hard/soft limits are 1048576 on other builders but x86_64/i686 has them at 1024. This seems to be the root cause of BuildStream/buildstream#1382

Description

Increase limits to at least 64k (recommendation by BuildStream devs), can be 1048576 for consistency.

Acceptance Criteria

Builds no longer randomly fail.