Improvements to code quality analysis in CI

This MR makes a few changes ('improvements') to the code quality analysis in the CI:

• always pass flake8, rely on the code quality report in the merge request UI
• improve parsing of requirements for flake8
• add dependency_scanning job

assigned to @duncanmmacleod

added 1 commit

• 4f7f2681 - ci: add dependency_scanning

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added 1 commit

• 6c728217 - ci: use igwn/builder in debian jobs

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mentioned in commit 80fc40ad

merged