ci: rename from Experimental to Nightly

[RobbieNeko]

Checklist

Required

• I wrote the PR title in conventional commit format.
• I ran the code formatter.
• I linked any relevant issues using github keyword syntax so it can be closed automatically.
• I have committed my changes to new branch that isn't main so it won't cause conflict when updating main branch later.

Purpose of change

People have continued to be scared of using the so-called "Experimental" builds, and thus running stables, often to their own detriment.

Describe the solution

• Renames the builds currently called "Experimental" to "Nightly" instead.

In my opinion, as well as the opinion of several others who provided their input as to the idea, Nightly is a far less intimidating name that fits our current build type perfectly. It still (to those familiar with the term, at least) comes with the connotation of a few bugs not being unusual to see, but without the heavy-handed "There WILL be bugs, and lots of them" that Experimental generally implies.
Additionally, it (generally) fits literally, as for many the builds are in fact done every night.

Describe alternatives you've considered

• Leave it be out of tradition
• Change "Stable" as well

I see less of a problem with Stable, as that one simply carries the connotation of being a version that's unchanging, as opposed to the ever-flowing regular builds. Sort of like Debian vs Fedora, to use a Linux analogy.

Testing

Test-ran the release on my fork, it seems to work fine (Android failing as usual, but that happens on literally every fork lol)
https://github.com/RobbieNeko/Cataclysm-BN/actions/runs/10977512821

Additional context

I didn't bother touching the solely-android commands / file paths because that looks a bit above my paygrade and is not something I could properly test myself anyway. For someone more familiar with the build process on Android, feel free to take the opportunity for a follow-up PR.

Also, feel free to change the type in the title. I had no clue which one exactly to select for such a change, and so I just went with CI on gut instinct.