fix: prevent codecov from publishing coverage report early

[StephenHulme]

Prevent CodeCov from publishing coverage report early

Changes proposed in this pull request

• Wait for 12 reports to be uploaded before CodeCov comments on PR

Instructions for Reviewers

See equivalent change in Limber: sanger/limber#1806

[All PRs] - Confirm PR template filled
[Feature Branches] - Review code
[Production Merges to main]
    - Check story numbers included
    - Check for debug code
    - Check version

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.45%. Comparing base (b4164ee) to head (73dc6b4).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #4269      +/-   ##
===========================================
+ Coverage    87.41%   87.45%   +0.04%     
===========================================
  Files         1369     1369              
  Lines        29601    29601              
===========================================
+ Hits         25876    25889      +13     
+ Misses        3725     3712      -13     

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[StephenHulme]

Note how there are no edits to the Codecov report above - compare with the report in #4265

There is still variability in the coverage - I suspect due to flaky tests - but at least now we get one number per PR

[sdjmchattie]

I think we could afford to be noisier about the warning in the ruby_test.yml file and make it more obvious what you mean in the warning, by using a substantial block of comment similar to:

########
## Warning! 
## CodeCov's config in `codecov.yml` is expecting an exact number of builds to complete before it will run.
## See the config value with key `after_n_builds` in that file.
## If the number of build steps changes, it is important to update the count there to match.
########

but I'm happy with the overall approach