chore: remove -Zmiri-tag-raw-pointers

This argument to `miri` is no longer necessary, as Miri now performs
the checks it enables by default. Passing the unstable argument is now
deprecated, so this ought to fix CI.

.github/workflows/ci.yml

@@ -250,4 +250,4 @@ jobs:
       - name: Run Miri tests
         run: cargo miri test --lib --no-fail-fast
         env:
-          MIRIFLAGS: -Zmiri-disable-isolation -Zmiri-tag-raw-pointers
+          MIRIFLAGS: -Zmiri-disable-isolation

bin/miri

@@ -6,5 +6,5 @@ set -x
 
 cd "$(dirname "$0")"/..
 
-MIRIFLAGS="-Zmiri-disable-isolation -Zmiri-tag-raw-pointers ${MIRIFLAGS:-}" \
-    cargo +nightly miri test --lib "${@}"
+MIRIFLAGS="-Zmiri-disable-isolation ${MIRIFLAGS:-}" \
+    cargo +nightly miri test --lib "${@}"