ci: twister: Check out west modules for test plan

This commit updates the twister workflow to check out the west modules
before running the test plan script for the pull request runs because
the twister test plan logic resolves the the module dependencies and
filters tests based on the local availability of the required modules.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>
(cherry picked from commit fad899d2)