So that it loads a consistent version of the library and `rubygems` is
never affected by gem activation conflicts related to `tsort`.

Getting CI green required updating one `bundler` spec, because `tsort`
is no longer loaded by `bundle clean` until after `BUNDLE_PATH` has been
changed, so to ensure it is found, it needs to be installed under
`BUNDLE_PATH` as well (which will be different from the global system
path on Bundler 3, meaning installing `tsort` to the global system path
is not enough there). This spec workaround can be removed once we also
vendor `tsort` inside `bundler`.

https://github.com/rubygems/rubygems/commit/d326880999