fix(string): use heap allocator for ZString.copy

Use the heap allocator for ZString.copy() instead of the temp allocator
to stay consistent across the code base.

The temp allocator is used for ZString.tcopy().

lib/std/core/string.c3

@@ -466,7 +466,7 @@ fn void String.free(&s, Allocator allocator = allocator::heap())
 
 fn String String.tcopy(s) => s.copy(allocator::temp()) @inline;
 
-fn String ZString.copy(z, Allocator allocator = allocator::temp())
+fn String ZString.copy(z, Allocator allocator = allocator::heap())
 {
 	return z.str_view().copy(allocator) @inline;
 }