CI: Add task that runs pgindent

At PGConf.dev 2025 one thing that came up in the "Scaling PostgreSQL
Development" unconference session is that new hackers don't know all the
details of our development flow by heart yet. Of course it's documented
on the wiki, but even if they find the relevant wiki pages they often
still miss/forget things. One of the things they often forget is
formatting their code. The consensus at that session was that it was
probably worth adding a CI task for this to nudge newcomers to indent
their code.

We're not too worried about this new requirement scaring away newcomers,
since autoformatting has become fairly commonplace in open source
development. Also committers can of course still choose to format the
patch themselves before committing if the formatting is failing.

This might also help reduce the number of unindented commits that
committers push, which require a follow up "fix indent" commit to make
the koel buildfarm animal happy again.