[Backport] Fix for currency update in crontab area

[denispapec]

Description (*)

This PR fixes issue where cron doesn't updates currency rates if they aren't enabled in default store.
This PR is for 2.2 Release line.

Fixed Issues (if relevant)

1. Currency rates not updated by crontab #18580: Currency rates not updated by crontab

Manual testing scenarios (*)

1. Base store has only one allowed currency (EUR). So EUR is the base currency.
2. English store has 4 allowed currencies (EUR, CAD, USD, GBP).
3. Enable Fixer.io conversion service and enable it.
4. Save manually incorrect rates for CAD, USD and GBP.
5. Run cron currency_rates_update
6. Confirm that all rates are updated to real values.

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

Backport for: #18981

[magento-cicd2]

All committers have signed the CLA.

[magento-engcom-team]

Hi @denispapec. Thank you for your contribution
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[jcourtei]

@denispapec Thanks!

[magento-engcom-team]

Hi @orlangur, thank you for the review.
ENGCOM-4660 has been created to process this Pull Request

[soleksii]

✔️ QA Passed

[m2-assistant]

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