Try to move phpstan-tmp out of the webroot

Problem/Motivation

Not really phpstan, more gitlab, but we don't have a component for gitlab.

We set the phpstan cache to core/phpstan-tmp so that it can be used as an artifact on gitlab, but this means it has to be excluded from copying around in some performance tests, e.g. #3612952: Optimize GenerateThemeTest.

If we can move it out of the webroot, we wouldn't have to account for it in tests.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

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