Remove nix caching on flake check CI

[benalleng]

This will remove nix caching, this also will serve as a way to observe how long the action takes without caching.

This is as a result of some out of band discussion with regards to the CI time.

Pull Request Checklist

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[coveralls]

Pull Request Test Coverage Report for Build 21071335593

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 83.058%

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Totals
Change from base Build 21044982881:	0.0%
Covered Lines:	9771
Relevant Lines:	11764

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💛 - Coveralls

[spacebear21]

cACK

For what it's worth this latest run https://github.com/payjoin/rust-payjoin/actions/runs/21080158957/job/60631686262?pr=1232 took roughly as long as the one in this PR. It looks like the cache was failing to initialize anyways, so I'm in favor of removing it until we find out a working solution.