[Merged by Bors] - chore(CI): fix merge conflict resolution for nightly-testing

[Garmelon]

Resolving merge conflicts in favor of nightly-testing when merging master may be convenient for nightly maintainers, but it can lead to changes made on master accidentally being dropped again later in the bump PR. For an example case, see #40189 (see #PR reviews > #39443 `nonempty_preimage_iff` @ 💬 for an explanation of what happened).

Resolving conflicts in favor of master means that nightly-testing will rather break than silently undoing changes made on master.

[github-actions]

PR summary 4eff6a086e

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 (regex)

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

You can run this locally as follows

## from your `mathlib4` directory:
git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci

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

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

The doc-module for scripts/pr_summary/declarations_diff.sh in the mathlib-ci repository contains some details about this script.

Declarations diff (Lean -- pending)

Computed after the build finishes.

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No changes to strong technical debt.

No changes to weak technical debt.

Current commit 4eff6a086e
Reference commit 034a5a7a3b

This script lives in the mathlib-ci repository. To run it locally, from your mathlib4 directory:

git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci
../mathlib-ci/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/nightly_merge_master.yml

[kim-em]

bors merge

[mathlib-bors]

Pull request successfully merged into master.

Build succeeded:

• ci (staging) / Post-Build Step
• ci (staging) / Lint style
• ci (staging) / Build
• ci (staging) / Post-CI job