[CI] Detect submodule changes using git status instead of git pull

[alyssadai]

Using the output of git pull run on the submodule main branch to signal if there are any new changes in the submodule doesn't really work because even as the submodule has updated to point to the latest commit, git pull will still show as Already up to date. It's only when we check the status of the parent repo (..) that we'll see changes in the data directory.

Current behaviour: even though the wf correctly detects uncommitted changes in the working directory after the Update submodule step (because it is looking for changes in the directory root) + then correctly opens a PR to update the submodule pointer, during the step, the script to regenerate figures is not triggered.

To resolve this, we use the output of git status in the root as the indicator of submodule changes instead.