Fix erroneous readonly on a mutable struct (#100996)

Author: stephentoub

src/coreclr/nativeaot/Runtime.Base/src/System/Runtime/TypeCast.cs

@@ -35,7 +35,7 @@ internal static class TypeCast
         private const int MaximumCacheSize = 4096; // 4096 * sizeof(CastCacheEntry) is 98304 bytes on 64bit. We will rarely need this much though.
 #endif // DEBUG
 
-        private static readonly CastCache s_castCache = new CastCache(InitialCacheSize, MaximumCacheSize);
+        private static CastCache s_castCache = new CastCache(InitialCacheSize, MaximumCacheSize);
 
         [Flags]
         internal enum AssignmentVariation