Add GOMEMLIMIT env variable

This MR adds GOMEMLIMIT for gitlab-zoekt-indexer to solve OOM issues when indexing large repositories (gitlab-org/gitlab-zoekt-indexer#11 (closed)). Our suspicion is that Go runtime doesn't free memory because GOMEMLIMIT is not set.

Steps to verify

1. helm install gitlab-zoekt . --set indexer.gomemlimit=100MiB
2. Ensure that kubectl describe pods gitlab-zoekt-0 returns

   Environment:
     GOMEMLIMIT:  100MiB

for zoekt-indexer