Miri flag passing changed

`cargo miri test` changed to be more compatible with `cargo test`, but this means we had to find a different way to pass flags to the interpreter, so there now is a `MIRIFLAGS` environment variable for that. The old way still works for now, but is deprecated.

This affects the following line:
https://github.com/brson/mem-basics/blob/804accc5a387b67290316b77cfb6f3a22123c00d/test.sh#L20
This should now be `MIRIFLAGS="-Zmiri-disable-isolation" cargo miri test -p memcpy --target-dir="$MIRI_DIR"`.

Open test.sh and inspect the command at line 20, along with the linked repository revision for context. Update the command's flag-passing form and run cargo miri test for the memcpy package to confirm the Miri test still works with the target directory setting.

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