Increase heap_size in .flowconfig to max for M1

Summary: Based on this comment, but would be great if @atul could test it out locally.

Test Plan: @atul tests it :)

Reviewers: atul, palys-swm

Reviewed By: atul

Subscribers: KatPo, zrebcu411, Adrian, subnub, karol-bisztyga, atul

Differential Revision: https://phabricator.ashoat.com/D703