add more to `nix flake check`

[peterbecich]

nix flake check currently: https://github.com/haskell/hackage-server/actions/runs/5983383840/job/16233463312

warning: ignoring untrusted flake configuration setting 'allow-import-from-derivation'.
Pass '--accept-flake-config' to trust it
warning: ignoring untrusted flake configuration setting 'extra-substituters'.
Pass '--accept-flake-config' to trust it
warning: ignoring untrusted flake configuration setting 'extra-trusted-public-keys'.
Pass '--accept-flake-config' to trust it
evaluating flake...
warning: unknown flake output 'haskellFlakeProjectModules'
warning: 'https://hydra.iohk.io/' does not appear to be a binary cache
copying path '/nix/store/7nilm17fcdna4zp6jcx7zgyf4mi0swl4-all-cabal-hashes-component-attoparsec-aeson-2.1.0.0' from 'https://hackage-server.cachix.org/'...
building '/nix/store/cldccmxpfcrbmg744w49xs8qsfr7ciq0-cabal2nix-attoparsec-aeson-2.1.0.0.drv'...
warning: The check omitted these incompatible systems: aarch64-darwin, aarch64-linux, x86_64-darwin
Use '--all-systems' to check all.

It also checks that nix flake develop can start successfully on Mac and Linux.

More Hackage tests could be included in nix flake check.

By default, all tests are included in nix build. Those tests can be disabled here: https://github.com/haskell/hackage-server/blob/613f5eb2f24d6ccbdef349d65f38247425ab64df/flake.nix#L21-L26