Re-enable Miri's float randomization in CI

[tomcur]

It seems Miri's approach to float error randomization has not fully stabilized yet (see, e.g., rust-lang/miri#4558, which as of writing is a week-old change). We pass a flag to Miri to disable randomization for now in CI. We'd like to remove this flag once Miri's approach becomes more stable.

See also #196.