[keyserver] Use build:debug instead of build in postinstall

Summary:
Context: https://linear.app/comm/issue/ENG-3213#comment-706a284f

Should reduce yarn cleaninstall time by a significant-ish amount.

Test Plan:
Didn't do any thorough profiling, but things appear to
A. be faster
B. be continuing to work as expected

Reviewers: varun

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D7021