builtins.storePath: Try to substitute the path if it is not yet valid

Signed-off-by: Shea Levy <shea@shealevy.com>
Bump language version for new storePath feature

This will allow e.g. channel expressions to use builtins.storePath IFF
it is safe to do so without knowing if the path is valid yet.

Signed-off-by: Shea Levy <shea@shealevy.com>