ci: get a list of changed files using git diff instead of GitHub APIv4

[jiridanek]

This approach is significantly slower than using the GitHub API (3s instead of 1s), but it is less magical for those unfamiliar with the API. So there are reasons to prefer this slower approach.

Description

Fixes #560

How Has This Been Tested?

• Draft PR to check that #602 changes work correctly in a PR that actually modifies some images #603

Merge criteria:

• The commits are squashed in a cohesive manner and have meaningful messages.
• Testing instructions have been added in the PR body (for PRs involving changes that are not immediately obvious).
• The developer has manually tested the changes and verified that the changes work

[openshift-ci]

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This pull-request has been approved by: caponetto, jstourac

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