Small CI refactor

This refactors our CI slightly, as part of #16 (closed).

Step	MR link
Refactor CI	👈 you are here
Publish latest scan result	!27 (merged)
Build basic dashboard	!24 (merged)

It's split into three commits:

ci: Only run scan-gitlab job in MRs

Its purpose is to make it easier to review rule changes in merge
requests, so there's little point running it on the main branch.

It now also only runs when there are changes to rules files.

ci: Extract "build combined yaml" job

The idea is to make the `pages` job simply responsible for copying files
prepared for it in `tmp` by jobs in its `needs` field.

It simply needs to know what files have been built, and where to copy
them to in `public`. How those files are prepared is the responsibility
of earlier jobs.

ci: Use spaces in job names

For consistency!

The only real change in behaviour here is in the first commit, which changes when a particular artifact is made available.