- 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>