fix: cache fails when Mathlib is a nightly dependency

[ineol]

Cache should on check for the nightly-testing remote when it is run from the Mathlib root and not from a repo which depends on Mathlib.

Fixes #28594

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I don't know how to test this fix since it would need to be in a nightly release, but at least it is working when ran from from tip of master.

[github-actions]

PR summary f4506f7151

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/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.

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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).

[ineol]

cc @kim-em

[robin-carlier]

Reassigning @kim-em as I’m not competent with the inner working of mathlib's CI infrastructure.

[kim-em]

I think this is not the right solution, and #30571 is better.