Fix constant name of `Ractor::IsolationError` message
`dest` of `const_decl_path` is `NODE_COLON2` or `NODE_COLON3` in some cases.
For example, `B::C ||= [“Not ” + “shareable”]` passes `NODE_COLON2`
and `::C ||= [“Not ” + “shareable”]` passes `NODE_COLON3`.
This commit fixes `Ractor::IsolationError` message for such case.
```
# shareable_constant_value: literal
::C ||= ["Not " + "shareable"]
# Before
# => cannot assign unshareable object to C (Ractor::IsolationError)
# After
# => cannot assign unshareable object to ::C (Ractor::IsolationError)
```