feat: test the cache as if in a downstream project

[kim-em]

Rather than just testing lake exe cache get && lake build --no-build, we run this test from a mini downstream project.

Sadly, we expect this to currently be failing on v4.16.0-rc1. I've tested that it works correctly on v4.15.0.

Having this test in place would have alerted us to the problems in v4.16.0-rc1 ahead of time.

[github-actions]

PR summary ab786c217f

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).