enable `-Zrandomize-layout` for miri tests

[hawkw]

we should be running Miri tests with RUSTFLAGS="-Zrandomize-layout". cordyceps and maitake currently contain code that perform layout-dependent casts (in this case, casting a ptr to struct to a ptr to the struct's first subfield), which would be UB if those structs were not #[repr(C)]. i believe that running the Miri tests with this flag would help catch any cases where a layout-dependent cast is inadvertently performed with a type that is not #[repr(C)] (or #[repr(transparent)]?), so we should be enabling it in our Miri CI jobs to catch any potential bugs involving this...