🌱 Add instructions to test cron controller + worker locally

[spencerschrock]

What kind of change does this PR introduce?

cron update

• PR title follows the guidelines defined in our pull request documentation

What is the current behavior?

It's difficult to test changes to the cron controller or worker because it relies on pub/sub and cloud buckets.
Given this, changes are primarily tested nightly as part of the release test pipeline.

What is the new behavior (if this is a feature change)?**

• Added new instructions in cron/internal/emulator on how to setup pub/sub and gcs fakes and how to run the scorecard controller / worker with the fakes.

• Standardize the controller/worker to using UTC time when determining the folder to use for shards/metadata. This is the behavior currently used by the cron in GCP, but without the behavior specified it wasn't working for me locally. The code ended up using a mix of my local timezone and UTC time, so the controller/worker were looking for different filenames.

• Force the worker to read the config file specified by --config. This should have been the case, but wasn't. Instead the workers have been using the config file embedded with the binary.

• Tests for the changes have been added (for bug fixes/features)

Which issue(s) this PR fixes

NONE

Special notes for your reviewer

Does this PR introduce a user-facing change?

For user-facing changes, please add a concise, human-readable release note to
the release-note

(In particular, describe what changes users might need to make in their
application as a result of this pull request.)

NONE

[spencerschrock]

@raghavkaul to test README instructions to ensure I haven't missed anything before converting this draft to a real PR

@calebbrown FYI. May affect existing controller/worker interactions for criticality_score depending on timezones used. May also be useful for your own testing.

[codecov]

Codecov Report

Merging #2817 (5f6386b) into main (df8b331) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

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[raghavkaul]

I was able to test the setup steps locally and confirm they work.