`pkgx --update` should update pkgs that were rebuilt but not bumped

[mxcl]

We use cache headers correctly so I think it is acceptable that we rebuild bottles in-place for bug fixes etc.

However the only way currently to get these updates is to delete the pkg first.

This will lead to a “delete everything and start again” approach to tea which we don’t want to foster.

Refs pkgxdev/pantry#1219