[CI] Install the test wheel last

[jjyao]

Why are these changes needed?

Make sure the test wheel is not overwritten by pip install ray[all].

https://beta.anyscale.com/o/anyscale-internal/projects/prj_2xR6uT6t7jJuu1aCwWMsle/app-config-details/bld_SxcxS3KMSFbUF2PSUB1Ln5hS

Related issue number

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(