fix test compilation and flake.nix

[peterbecich]

I have not fixed the failing tests:
13 tests and 2 failures

1 failure might be easy to solve by providing purs 0.14 in flake.nix or updating the package set; I will try to do this later

       installed `purs` version:    0.15.7
       minimum package-set version: 0.14.3

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I am attempting to upstream this fork to eskimor#63, I hope that is okay

Thanks

[peterbecich]

Incidentally, if this fork is targeting Purescript 0.14, the upstream is targeting 0.15; maybe some work can be saved