Allow specifying a subset of actions to run in CI

[evansd]

Supports running just a subset of pipeline actions rather than run_all e.g.:

 - name: Test that the project is runnable
   uses: opensafely-core/research-action@v2
   with:
     actions: test_action_1 test_action_2 test_action_3

Docs added in:

• Document the new argument to the research action opensafely/documentation#1745

Closes #112