ci: auto-update mathlib-ci ref

[bryangingechen]

This adds steps to update_dependencies.yml to keep the default ref in the get-mathlib-ci action up to date.

prepared with claude code

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Test runs:

• with no mathlib CI differences, the check returns false: https://github.com/leanprover-community/mathlib4/actions/runs/23850751741/job/69529890461#step:11:27
• with mathlib CI differences, the check returns true: https://github.com/leanprover-community/mathlib4/actions/runs/23851062508/job/69531011033#step:11:39

[github-actions]

PR summary 63486b57ca

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

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Declarations diff

No declarations were harmed in the making of this PR! 🐙

You can run this locally as follows

## summary with just the declaration names:
./scripts/pr_summary/declarations_diff.sh <optional_commit>

## more verbose report:
./scripts/pr_summary/declarations_diff.sh long <optional_commit>

The doc-module for scripts/pr_summary/declarations_diff.sh contains some details about this script.

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No changes to technical debt.

You can run this locally as

./scripts/reporting/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).

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⚠️ Workflow documentation reminder

This PR modifies files under .github/workflows/.
Please update docs/workflows.md if the workflow inventory, triggers, or behavior changed.

Modified workflow files:

• .github/workflows/update_dependencies.yml