ci: add reusable installer XML change check

[karthikbekalp]

What was the problem/requirement? (What/Why)

Some individual submitter installer changes were missing from the shared installer. This check reminds authors to review whether the shared installer also needs an update.

What was the solution? (How)

Add reusable_installer_xml_check.yml with a small Python check. It uses Git to compare the PR's changes.

Adding, editing, deleting, or renaming an XML file under installer/ or install_builder/ fails the check, including nested files and formatting-only edits. With no matching XML changes, it passes.

The log lists the changed files and shows:

Installer definitions changed. Please verify whether any related internal changes are needed to reflect the changes in this PR.

What is the impact of this change?

To enable the check:

1. Merge this PR.
2. Add .github/workflows/installer_xml_check.yml to each submitter repository included in the shared installer, using the example in .github/installer-xml-changes.md.

Each repository adds the workflow once. Future updates come from @mainline. It runs on PRs, including drafts, separately from Security Scan and Code Quality.

When it fails, review the listed files and check whether the shared installer needs an update. Arrange any follow-up and confirm in the PR discussion: "Follow-up is complete", "Follow-up is under review", or "No follow-up is needed". Keep private details and links out of public discussions.

This is an optional reminder. It stays failed after confirmation while the XML change is in the PR. It does not verify updates to the shared installer. The job has read-only access and does not run PR code or installer definitions.

Dependent draft PRs

Merge this PR first, then these eight submitter workflows:

• Maya #487
• 3ds Max #301
• Cinema 4D #585
• After Effects #348
• Nuke #368
• VRED #165
• Blender #400
• Houdini #403

How was this change tested?

• All 58 existing repository tests passed.
• The Python check passed 17 Git-based scenarios covering edits, renames, unrelated files, target-branch changes, Git errors, and safe filename reporting.
• Cinema 4D test PR #584 used the same Python check and added one XML comment. The GitHub Actions run ran on the draft PR, found the changed XML file, and failed as expected with the reminder and exit code 1. The test PR is closed without merging.
• The workflow and documented caller passed actionlint and zizmor validation.
• git diff --check passed.

Was this change documented?

Yes, in .github/installer-xml-changes.md, with setup instructions and what to do when the check fails.

Is this a breaking change?

No. Each repository chooses whether to add the workflow.

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