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/tests/interpreter/mod.rs

@@ -620,6 +620,19 @@ fn yaml_test_impl(file: &str) -> Result<()> {
         }
     }
 
+    #[cfg(miri)]
+    {
+        // Skip tests with large-exponent Number comparisons that hit a
+        // Float-vs-BigInt representation mismatch under Miri's soft-float.
+        let skip = ["units/parse.yaml", "units/parse_bytes.yaml"];
+        for s in skip {
+            if file.contains(s) {
+                std::println!("skipped {file} under miri.");
+                return Ok(());
+            }
+        }
+    }
+
     std::println!("running {file}");
 
     let v0 = !file.contains("bindings.yaml");