Tighten CI - #856

BTW I also made XhmikosR/oxipng@xmr/ci...XhmikosR:oxipng:xmr/hash (I used AI for the rust code), but it seems to work and will fix #727. It's built on top of this branch so I haven't opened it yet, but you can check the binaries from my fork.

Rebased and pinned all actions. I haven't tested the release workflow like I said in the PR description, but if something doesn't work feel free to CC me later and I'll have a look at it :)

The clippy issue I tried to fix it in another branch by not using the nightly toolchain so that it doesn't randomly break CI. But that;s for another time...

I think using the nightly Rust toolchain is fine for this project, since the breakage will happen sooner or later anyway, and it's not such a big chore to fix the lints as they appear. I'll do that right now on the main branch.

@AlexTMjugador Should we do another release due to the man page issue? And to test the new workflows.

Should we do another release due to the man page issue? And to test the new workflows.

Sure, that sounds sensible to me! It's quite late where I am though, so if anything bad happens now with the new workflows I won't be available to immediately take a look at it.