Use an absolute path when loading plugin routes
Use an absolute path when loading plugin routes

Our current way of using a relative path means that
these pre_routes are not available if you run a
rails command from a directory other than the root.

refs INTEROP-7615

flag = none

test plan:
- Go to the root directory in your docker container, or
  any dir besides the actual rails project root
- Run `DISABLE_SPRING=1 /usr/src/app/bin/rails c`
- Run:
  ```
  Rails.application.routes.routes.map { |route|
    route.path.spec.to_s
  }.grep(/search/)
  ```
- verify that /api/v1/accounts/search is one of the
  results
  - Note: that is different from
    /api/v1/accounts/:account_id/search

Change-Id: Ia5332f656c0ef398f2ef1273affe15aceae86a25
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/300034
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Jacob Burroughs <8ecea6e385af5cf9f53123f5ca17fb5fd6a6d4b2@instructure.com>
QA-Review: Tucker Mcknight <4b51b98c1d84aa97854d216bd061ce5d3eda8a28@instructure.com>
Product-Review: Tucker Mcknight <4b51b98c1d84aa97854d216bd061ce5d3eda8a28@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms