fix(core): skip fast cache migrations check for anonymous users

[Panaetius]

We can't do the gitlab api path for users that aren't logged in, since we don't have a gitlab token for those.

[coveralls]

Pull Request Test Coverage Report for Build 5822346792

• 5 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• 400 unchanged lines in 39 files lost coverage.
• Overall coverage increased (+0.9%) to 85.7%

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Files with Coverage Reduction	New Missed Lines	%
renku/command/schema/dataset.py	1	92.37%
renku/core/dataset/providers/api.py	1	83.26%
renku/core/dataset/providers/common.py	1	94.44%
renku/core/dataset/providers/repository.py	1	88.46%
renku/core/dataset/providers/s3.py	1	96.25%
renku/core/dataset/providers/web.py	1	89.91%
renku/core/migration/m_0003__1_jsonld.py	1	87.68%
renku/core/migration/models/v3.py	1	97.24%
renku/core/storage.py	1	79.79%
renku/core/util/os.py	1	88.89%

Totals
Change from base Build 5819037706:	0.9%
Covered Lines:	25817
Relevant Lines:	30125

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💛 - Coveralls