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>

.github/workflows/miri.yml

@@ -10,6 +10,8 @@ jobs:
   miri-test:
     name: miri (nightly)
     runs-on: ubuntu-latest
+    env:
+      MIRIFLAGS: "-Zmiri-disable-isolation"
     steps:
       - name: Checkout repository
         uses: actions/checkout@v4