chore: update `flake.nix` to unpin `matchers`

[ggreif]

core is now included, so run latest motoko-matchers with a core package.

While matchers internally uses core, it has been retrofitted to work with base too. So we can now test both core and base with matchers.

[github-actions[bot]]

Comparing from 644fbdb814ed354b115e855d747666b1b8673eaf to b1374511807e0e632a33374c6cef198ec3e35f19: The produced WebAssembly code seems to be completely unchanged. In terms of gas, no changes are observed in 5 tests. In terms of size, no changes are observed in 5 tests.

[christoph-dfinity]

[email protected] switches from base to core. As this code is running bases test-suite it should probably stick to [email protected].

[ggreif]

[email protected] switches from base to core. As this code is running bases test-suite it should probably stick to [email protected].

I don't quite understand. It appears that [email protected] easily can test base (see https://github.com/dfinity/motoko-base/pull/736). The relevant functionality should now be present.