Fix handling of default_install_tag #372

As noticed in these logs, install.default_install_tag is overridden by the default configuration to 3, rather than matching the value of default_tag. This means that a user who has overridden their default_tag to (say) 3.13 can't use py install default properly, because it will infer that to mean 3 and so will match any existing 3.x install rather than the intended default.

We either need to remove the default value for install.default_install_tag (so that it uses default_tag instead) or change when it's used (e.g. not for literal py install default but keep it for other default installs).