Adjust padding for Debian component file size

(Description stolen from !110679 (merged))

🃏 Context

In #389950 (closed), it has been discovered that size of Debian main/binary-amd64/Packages can vary around 1000, i.e. from 3 to 4 digits.

See more details in #389950 (closed)

This creates flaky specs. This MR updates the expectations to ensure the padding is correct.

🤔 What does this MR do and why?

• Updates spec/support/shared_examples/services/packages/debian/generate_distribution_shared_examples.rb to adjust padding for Debian component file size to always 8 characters

📺 Screenshots or screen recordings

n / a

⚙ How to set up and validate locally

$ bundle exec rspec spec/workers/packages/debian/generate_distribution_worker_spec.rb:60

$ bundle exec rspec spec/services/packages/debian/generate_distribution_service_spec.rb:18

🛃 MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.