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>

src/utils/limits/mod.rs

@@ -6,7 +6,7 @@
 #![allow(dead_code)]
 
 mod error;
-#[cfg(feature = "allocator-memory-limits")]
+#[cfg(all(feature = "allocator-memory-limits", not(miri)))]
 mod memory;
 mod time;
 
@@ -14,7 +14,7 @@ mod time;
 pub use error::LimitError;
 
 #[allow(unused_imports)]
-#[cfg(feature = "allocator-memory-limits")]
+#[cfg(all(feature = "allocator-memory-limits", not(miri)))]
 pub use memory::{
     check_global_memory_limit, enforce_memory_limit, flush_thread_memory_counters,
     global_memory_limit, set_global_memory_limit, set_thread_flush_threshold_override,
@@ -34,19 +34,19 @@ pub use time::acquire_limits_test_lock;
 #[allow(unused_imports)]
 pub use time::{set_time_source, TimeSourceRegistrationError};
 
-#[cfg(feature = "allocator-memory-limits")]
+#[cfg(all(feature = "allocator-memory-limits", not(miri)))]
 #[inline]
 pub fn check_memory_limit_if_needed() -> core::result::Result<(), LimitError> {
     memory::check_memory_limit_if_needed()
 }
 
-#[cfg(not(feature = "allocator-memory-limits"))]
+#[cfg(any(miri, not(feature = "allocator-memory-limits")))]
 #[inline]
 pub const fn enforce_memory_limit() -> core::result::Result<(), LimitError> {
     Ok(())
 }
 
-#[cfg(not(feature = "allocator-memory-limits"))]
+#[cfg(any(miri, not(feature = "allocator-memory-limits")))]
 #[inline]
 pub const fn check_memory_limit_if_needed() -> core::result::Result<(), LimitError> {
     Ok(())