[Merged by Bors] - fix(CI): run leanchecker in mathlib4 repo for both master and nightly

[kim-em]

This PR fixes the daily CI workflow for leanchecker.

The check-leanchecker job was configured to run only in mathlib4-nightly-testing, but the notify-leanchecker job runs in mathlib4 and tries to query the job status from the current workflow run. Since the job was skipped in mathlib4, the notification produced broken Zulip messages with empty URLs (see https://leanprover.zulipchat.com/#narrow/channel/428973-nightly-testing/topic/leanchecker.20failure/near/570212817).

This PR:

• Moves check-leanchecker to run in mathlib4 (matching check-nanoda and check-mathlib_test_executable)
• Enables both master and nightly branches for leanchecker (the TODO comment indicated this was waiting for leanchecker to land on master)

🤖 Prepared with Claude Code

[github-actions]

PR summary 0e16bffea0

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

---

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/declarations_diff.sh <optional_commit>

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

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

---

No changes to technical debt.

You can run this locally as

./scripts/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).

[bryangingechen]

Thanks!
bors r+

[mathlib-bors]

Pull request successfully merged into master.

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