zoekt: use GOGC=25

Created by: keegancsmith

After much experimentation we have a new value for GOGC, which is 25. We noticed a large regression in zoekts memory use when we switched to go1.18. So we reverted to go1.17 in zoekt releases and since then have been experimenting with different GOGC values. We have found the same stable heap size with GOGC=25 and have had this stable size for more than a week.

• https://github.com/sourcegraph/zoekt/commit/42276b7b70 docker: change our default GOGC to 25
• https://github.com/sourcegraph/zoekt/commit/267622ccb4 fix constant evaluation of empty repo set

Test Plan: Lots of experimentation in our production work loads has been done. This is the outcome of that.