circleci: Fix CircleCI OOM errors

[tamirms]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with name of package that is most changed in the PR, ex.
  services/friendbot

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.
• I've updated any docs (developer docs, .md
  files, etc... affected by this change). Take a look in the docs folder for a given service,
  like this one.

Release planning

• I've updated the relevant CHANGELOG (here for Horizon) if
  needed with deprecations, added features, breaking changes, and DB schema changes.
• I've decided if this PR requires a new major/minor version according to
  semver, or if it's mainly a patch change. The PR is targeted at the next
  release branch if it's not a patch change.

Summary

Occasionally CircleCI jobs triggered by PRs fail because of OOM errors (see https://circleci.com/gh/stellar/go/6268#usage-queue/containers/0 for an example). The cause and fix for this issue is outlined here:

golang/go#26186 (comment)

[accordeiro]

Nice catch! :)

[bartekn]

Great, thanks!