Add troubleshooting command to miri docs (#5116)

# Objective

When `miri` runs in our build system to detect unsoundness, its output can be very unhelpful, as the tests are all run in parallel.

## Solution

Add a comment documenting the extremely obvious 10/10 command used by @BoxyUwU in #4959.

I've stuck this in the CI file, as it seems like the most obvious place to check when frustrated. I didn't put it  in CONTRIBUTING.md because this is an eldritch abomination and will never be useful to new contributors.

Author: alice-i-cecile

.github/workflows/ci.yml

@@ -93,6 +93,8 @@ jobs:
       - name: Install alsa and udev
         run: sudo apt-get update; sudo apt-get install --no-install-recommends libasound2-dev libudev-dev libwayland-dev libxkbcommon-dev
       - name: CI job
+        # To run the tests one item at a time for troubleshooting, use
+        # cargo --quiet test --lib -- --list | sed 's/: test$//' | MIRIFLAGS="-Zmiri-disable-isolation -Zmiri-permissive-provenance -Zmiri-disable-weak-memory-emulation" xargs -n1 cargo miri test -p bevy_ecs --lib -- --exact
         run: cargo miri test -p bevy_ecs
         env:
           # -Zrandomize-layout makes sure we dont rely on the layout of anything that might change