rename GL_TOKEN_READ_REGISTRY to GL_TOKEN_SCANNER_READ_API

https://gitlab.com/gitlab-org/gitlab/-/issues/460398+s

Following this comment

READ_REGISTRY in the name of this build parameter is misleading.
It is NOT supposed to contain a token with the read_registry permission.

The purpose of this build parameter is to provide a token with read_api permission on the scanner project.

For that reason, it will be renamed GL_TOKEN_SCANNER_READ_API