CI: Miri flag passing needs to be fixed

[RalfJung]

With rust-lang/miri#1769 having landed, this way of passing Miri flags will soon stop working:

crunch-lang/.github/workflows/main.yml

Line 255 in e51945a

cargo +nightly miri test -- -Zmiri-disable-isolation

This should now instead be

MIRIFLAGS="-Zmiri-disable-isolation" cargo +nightly miri test

[Kixiron]

Fixed in b4ef3fc, thanks!