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]

Comparing from bbe13bd to 0efe8c1:
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 dfinity/motoko-base#736). The relevant functionality should now be present.