Enable -Zmiri-retag-fields in CI

Now that the bug is fixed, this can be enabled. Field retagging is
required to justify some of the `noalias` optimizations that rustc
performs currently.

Author: Noratrieb

ci/miri.sh

@@ -9,4 +9,4 @@ rustup toolchain install nightly --component miri
 rustup override set nightly
 cargo miri setup
 
-cargo miri test
+MIRIFLAGS='-Zmiri-retag-fields' cargo miri test