Problem: we've recently supported ghc-9.10.1, but it's not tested
by CI. Also we're still testing ghc-8.8.4, which is rather old and
causes some troubles when updating Nix dependencies.

Solution: add 9.10.1 to tested GHC versions, remove 8.8.4.

hackage     a62dac66a9 (2024-05-13) -> 5153fdaef0 (2024-07-08)
haskell-nix 28fceff2ef (2024-01-10) -> 34946405e8 (2024-06-21)
nixpkgs     sha256-dd/ (1970-01-01) -> fe065f84d7 (2024-07-01)
stackage    207b7104d5 (2024-05-12) -> c84e07a4e0 (2024-07-06)

Problem: CI is triggered only on push, so it won't work in case of
PRs from forks.
Solution: trigger it on PRs as well. Trigger on push only when the
branch is master.
If you want to run on push to your feature branch, you can edit
check.yml in your branch.