main thread random context doesn't reinitialize correctly with non-weak source

TLDR is that `_mi_auto_process_init` attempts to reseed the main thread's random ctx (which would cause BCrypt to get loaded there) but there's a bug preventing that from working.

Starting point:

https://github.com/microsoft/mimalloc/blob/09a27098aa6e9286518bd9c74e6ffa7199c3f04e/src/init.c#L580

`_mi_auto_process_init` calls `mi_heap_main_init` which on `_WIN32 && !MI_SHARED_LIB` builds calls `_mi_random_init_weak` for bcrypt-related reasons.

Then, we call `_mi_random_reinit_if_weak` at the end of `_mi_auto_process_init`

https://github.com/microsoft/mimalloc/blob/09a27098aa6e9286518bd9c74e6ffa7199c3f04e/src/init.c#L600-L601

BUT, that will never actually do anything.

Why? The `mi_random_ctx_t` `weak` bool will _always_ be false after any call to `mi_random_init_ex`.

Why? At the end of `mi_random_init_ex`, for either the weak or normal path, we call `chacha_init`.

And what does that guy do? Memsets the entire `mi_random_ctx_t` to zero, wiping out the `weak` bool.

https://github.com/microsoft/mimalloc/blob/09a27098aa6e9286518bd9c74e6ffa7199c3f04e/src/random.c#L97-L102

We discovered this when we saw a thread was doing the mimalloc BCrypt-related initialization (e.g. LoadLibrary) during the initialization of a thread, which involved calling `LoadLibrary` a couple times and holding an internal BCrypt critical section while the thread initialization has the loader lock held (because the root `calloc` in that new thread is inside of `__vcrt_getptd_noexit`). So it seems, given the above mentioned code, that it's not intended that we'd be leaving the main thread's initialization without initializing the BCrypt stuff?

Start in src/init.c at _mi_auto_process_init and trace the calls to mi_heap_main_init and _mi_random_reinit_if_weak, then inspect src/random.c around mi_random_init_ex and chacha_init. Check the Windows non-shared-library initialization path and verify that the main-thread random context is correctly reinitialized without triggering the problematic BCrypt work during thread initialization.

Written by the indexing model from the issue text.