gomod: bump zoekt to prevent multi-writers on index dir

Created by: keegancsmith

Includes commits:

• https://github.com/sourcegraph/zoekt/commit/3bb00e7 indexserver: assert and check for ownership on index directory
• https://github.com/sourcegraph/zoekt/commit/ec4be9d tests: make the code pass the go vet checks
• https://github.com/sourcegraph/zoekt/commit/cdb5037 all: minor refactoring

Test Plan: CI