fix(ci): skip mimalloc FFI and disable isolation for Miri (#621)

- Add cfg(not(miri)) guards to mimalloc module, global allocator, and
  allocator-memory-limits code paths so Miri falls back to the default
  system allocator instead of calling unsupported FFI functions.
- Set MIRIFLAGS="-Zmiri-disable-isolation" in the workflow so tests
  that perform file I/O can run under Miri.
- Skip units/parse tests under Miri due to Float-vs-BigInt Number
  representation mismatch with Miri's soft-float emulation.

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

tests/memory_limits.rs

@@ -1,4 +1,4 @@
-#![cfg(all(feature = "mimalloc", feature = "allocator-memory-limits"))]
+#![cfg(all(feature = "mimalloc", feature = "allocator-memory-limits", not(miri)))]
 
 use std::sync::{Mutex, OnceLock};